This hasn't happened to Poe since he was much younger. Since, at puberty, he'd learned to control it like all his subspecies. It was a genetic variance passed on through a line, a survival adaptation that had been programmed in sometime long ago when his father's ancestors had faced extinction. Of course, in his youth, it had been very hard to see just what use it was to turn into an aquatic mammal when one lived, primarily, on a jungle planet, but the device in his genetic code seemed to pre-date galactic travel. Besides, for all other intents and purposes he was human.
And while it was very helpful to have webbed feet, a long sleek body, sharp teeth, and water repellent fur from the tops of his small round ears to the tips of his toes - set at the end of short powerful limbs - when one was say, drowning (as now), it was far less helpful when one was piloting an X-Wing in cold, airless space.
Put simply, since joining first the New Republic and then the Resistance, Poe has had no use for this form. That is, until now. Until he finds himself plunged into slimy, murky water on a strange planet, sinking like a stone in all of his flight gear after fleeing the site of a spectacular (don't say crash!) landing on some far away planet. He doesn't know how deep he's gotten, or where the surface is, or how far he was thrown from the water's edge. Instinct takes over, and the transformation overcomes him, changing his body - it doesn't hurt, not quite, but it feels like stretching just a little too hard. Like lifting your arms over your head just a little too far,and pushing the curve of your spine to a fraction past comfortable.
Then he slips free of his clothes, lithe and buoyant, body too slim and limbs too squat to hold the flight suit on. He doesn't stay to watch it sink, instead cupping the water with broad, webbed paws and going in the direction he can feel the air in his lungs pulling him. It feels urgent - not quite to bursting, but he wants fresh air, wants to take a breath, hopes after paddling what seems like a very long time that his instincts are right, not jogged by going unused too long or shaken by the crash.
Then light begins to penetrate - and he can hear, somewhere, Finn screaming for him. His round head breaks the surface, and the air feels warmer than the water, tastes like murky swamp when he gasps it in. There is some algae on the surface that coats him unpleasantly, sticking in his fur.
Finn screams his name again. Poe turns, eyes on level with the water, and looks for him.
In retrospect, when Poe climbs all the way out of the water coated in algae, a nearly-man-sized sea creature from the depths dripping and sleek and probably grinning (he tends to grin fairly uncontrollably after he's been in life threatening danger) and full of enthusiasm for his partner, there was a point in their relationship before this point when Poe should have mentioned this.
Finn just screams this time, aiming his blaster.
"No, its' really me," Poe assures him, for the twelfth time. "Can we discuss this another time?"
"Where are your clothes?" Finn demands. His eyes are wide, wild.
"Probably at the bottom of the lake?" Poe gestures. The sound of blaster fire is very near now.
"Are you going to change back into that thing?" Finn asks. "And go get them?"
Poe looks down at himself. Not even his boxers had survived the swim. He looks back at the algae-covered and stinking surface of the water. No way in all the galaxy...
"Nope," he says cheerfully. "And what do you mean, 'that thing' - I think I'm pretty cute either way."
"Cute? Have you seen the size of your teeth?"
It's not the worst reaction he could be getting, he supposes, and given time to calm down, to examine the situation without stress hitting him from all angles, Poe knows Finn will feel more curiosity than threat. The thought of strong, dark fingers scratching the fur just behind his ears... well now Poe is getting off track and the situation isn't getting any better.
"Well, I think it's pretty safe to say we found a First Order outpost out here," Poe says, trying to steer them back toward a useful topic of conversation. "Don't you think we should focus on-"
"Finding your pants?"
A crashing in the undergrowth nearby - at the edge of the lake - suggests an immediate approaching threat. Poe yanks the blaster out of the holster at Finn's hip, trusting Finn to defend himself with the plasma rifle he was carrying. It's drafty, sure, but at least it's dark. Maybe the stormtroopers won't laugh too hard at Poe's exposed, er, position, if he kills a few before they figure it out.
Instead, first a seven foot tall creature that's even hairier than Poe in his alternate form and a slight girl in tan robes burst through into the clearing at the edge of the lake, going full tilt.
The rescue party. Of course.
"Do you really," Rey demands immediately, looking faintly out of breath, beautiful (like always), and angry all at the same time. In one hand, she has a lightsaber. In the other, raised to keep the muzzle pointed safely at the sky, is the blaster Han Solo had given her. "that now is the appropriate time for-"
Finn starts protesting before she even can finish. "It's not that - It's not that,"
"Then what," she starts to ask.
"I know what it looks like," Finn continues. "And it isn't that at all"
Poe can't even bother to be embarrassed. The sounds of pursuit behind their rescue party are fairly alarmingly close."
"So what were you doing out here naked?"
"Only he's naked," Finn points out.
"Very naked," Poe agrees. "And very willing to find someplace to retreat to before-"
Neither pays any attention to him.
"Look, I know you two love a thrill," Rey says, sounding exasperated. "Don't think I didn't know what happened in the pilot seat of my ship, but this - don't you think this is-"
"It's not - we weren't - it's just that - tell her Poe!" Finn says. Then, before Poe can do anything of the sort. "He turned into a big scary monster."
Poe can feel his expression turning to hurt indignity, and he turns his palms up toward Finn in disbelief. "Scary?"
"He was in the lake - I thought he was going to drown, and then he came out as a-"
"It's not scary," Poe protests.
"Maybe not scary," Finn agrees, somewhat reluctantly.
"Look," Poe says. "I'll show everybody everything that they haven't already seen of mine - and at this exact moment that isn't much - whenever they want when we are all safe and out of here!"
Later he thinks, bolting naked through the thick forest undergrowth, and then again with his bare ass sticking uncomfortably to the pilot seat of another stolen TIE fighter - that maybe he should make sure none of them have any more surprising secrets. Who knew what a good long 'show and tell' session might reveal, between the three of them?