The Ocean is pretty wet, and salty - the combination of these two things can cause big problems, especially to electronics.
Ngalawas are really wet too, they leak even when they aren’t swamped - which they will probably be at some point. Think windsurfer wet, rather than yacht wet and you’ll get the idea.
There are a few tips and tricks you could try, like keeping some air inside the bag to create an outward pressure; double lining; cling wrapping; condoms; etc. Probably a good idea to test your choice in the bath before trying it in the drink.
Alternatively, we would recommend investing in some high quality dry-bags. Watershed are an excellent brand, and they're offering a 25% discount for NC teams.