In your scenario, is this person jumping into the water or just gradually sinking in?
If it's the former, consider the stages of what's happening to your character:
- We hear above water ambiences as you establish that scene
- There's a sharp crack as the body enters the water, while the ears are still above it
- That initial crack blends into a more muted, rushing water sound as the ears plunge underwater
- There's an ambience transition as the above-ground sounds basically disappear (except for any particularly loud / transient ones, like a clicking generator or something)
- There's extra bubbling/fizz that rushes up as all the water around the character kind of settles once the movement's done
If you have access to any decent libraries, I would really focus on finding the 'right' splash, making sure it has a good stereo width to it and feels more like a first-person splash than the deep ker-SPLUNK of a rock getting tossed into a lake from a different microphone. That's going to be the biggest piece that sells the first-person perspective.
Sweeping a low-pass filter (at 6-12dB per octave) down really quickly over some of the ambiences at the moment of submersion will help sell things, too. But it definitely won't get you all the way there so you should really have a totally separate bed of underwater ambiences that you can fade up.
I dunno, I've generally found that things recorded underwater with a hydrophone rarely sound like that cinematic underwater sound we've come to expect. What you're really going to be looking for is the sound of water rushing from an underwater perspective, to play at that moment when you sink under.. I got some good results out of using a hydrophone in a toilet flush, you may not want to go that far.
Also try ducking your head underwater (briefly, don't drown on us) and just counting up the layers of sound you hear. Try to identify stuff in each frequency range and get ideas for reconstructing them later. I'm almost positive there will be elements -- that omnipresent low rumbling, and the sound of bubbles, that you'll need to layer in there to sell the scene, in addition to whatever you want to record yourself.
That all should be a reasonable start -- let me know how it goes, and good luck!
