Dropped iPhone in sea

Yeah - back it up because you never know if something might happen to it.

However, more often than not the water resistant seals hold, but no telling how many times or if there are random differences where it might not hold again. Apple makes no guarantees that they won't suffer damage when they get wet, and salt water is especially bad.

When you're sure that it's dry, you might try looking in the SIM card slot to see if the liquid contact indicator turned red. If it (or any other LCI inside) did, Apple' generally won't provide a same-device service any more.

Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

Back it up IMMEDIATELY. As sberman said, it could go down at any time. Prepare in every way possible. It's ok to use it, but its very important to back it up.

Expect a catastrophic result at any time.