I'm sure Surf will always be there. HMs 1-4 have always been the same.
I believe the water encounter rate should be the same, maybe lowered if the region has a lot of water like that of Hoenn. Speaking of Hoenn, it had the best sea sections IMO: currents, dive spots, different things to find. Dive should return and be more useful unlike in Gen 5, more like in Gen 3. Underwater battles would be cool. Maybe even a lost underwater city that has something the evil team of this game wants.
I've always liked Whirlpool from G/S/C/HG/SS, I'd like Whirlpool as maybe a TM that you can use to find alternate pathways and cool items, much like HMs were used for in BW. I'd also like HMs to be required again in order to progress in some areas tbh.
Another thing I'm wondering is how you will look when you surf. Will you be surfing on a whale head as always?
As long as surfing is not as slow as it was in D/P...
I think it'd be interesting for humans to be able to swim themselves tbh. I mean you could take swimming lessons (which'd look awesome to actually DIVE into the water given that it's 3D and you could see the sun as you dive in (to swim, not to dive) and just... swim along! Ugh I want this. I CAN TOTALLY SEE IT. :(