Probably either a dolphin or a shark if I wanted to go by sea - something big enough to at least have a little visibility and idea of which direction I was headed in. Each have their pro's and cons - dolphin could echo-locate but you have to keep coming up for air; shark you wouldn't have to come up for air, but goodness help you if you find another person sinking and dying along the way, I don't know if I could live with myself if I was a shark and ate someone, and there's no sonar.
Seagulls aren't reliable, in my mind. They're flappers - I'd be too afraid of getting tired in morph. Pelican would be better, but, if the oil on the sea is burning, there's a chance that you could still suffocate on the black smoke oil puts off when it burns before you could get very far - you'd breathe in too much, pass out, and be a bird face-down in the water, if you made it past the burning oil spill.
I'd definitely be more comfortable going underwater than above - on would be certain death, so...I think I'd have to go with Dolphin.