I think the technicality here is that it defines "oceans" not "water" in general.
Here is the water domain:
forgottenrealms.fandom.com/wiki/Water_domainand the ocean:
forgottenrealms.fandom.com/wiki/Ocean_domainSo they are similar to some extent but there is a difference and one is quite a bit more specific than the other, that being the ocean. Water more specifically deals with the turning of elementals and having power over that while the ocean domain simply makes one more adaptable in that environment - able breath underwater.
Could the water domain entail the ocean? Probably. Could the ocean domain encompass what the water domain grants? I think that's debatable since it just doesn't seem as broad. The other thing is that the water domain spells(in nwn's) don't really lend RP wise to Ulu much...
Water Domain gained spells:
Level 3 Poison
Level 5 Ice storm
Also if you put the Water domain up against the protection domain they kind of contradict one another in light of what Ulu stands for. Ulu is much more about respect for animals and their protection. True he is a god for polar environments but this in no way suggests he's trying to whip up storms, freeze everything to death, flood things, torture, slay, or anything of that nature. While Ice Storm could fit into his portfolio he isn't exactly a deity of fury like Auril is who relishes in those kinds of happenings and poison really doesn't fit Ulu at all.
I think for these reasons Ocean is different enough compared to the Water domain and water probably shouldn't be taken as a substitute.
If this were a situation where this deities clergy were much more mechanically limited due to nwn's domains then I'd be more inclined to saying yes and letting this slide but clerics of Ulu have a decent choice of domains to choose from without trying to make water represent Ocean.