First off I would like to agree with ginganinja with respect to our Ground-type weakness. That weakness is a huge part of who we are, and what we want to threaten us, and eliminating it completely changes that up. Not to mention how questionable it would be with respect to the concept itself.
With that said, I really do want to try and re-emphasize what DLC asked in his opening post. We have had a lot of ability suggestions, but not really much discussion of his actual question. What exactly do we want this ability to accomplish? I have seen plenty of posts suggesting abilities that help us against Water-types, for example, but if we want to deal with water types through an ability, do we really want to do it in the base form? Or is that something that is best accomplished in the mega form, that we may be in for most of the battle.
Additionally, I would like to remind people not to assume too much with regard to abilities, with the big example here being Drought. Drought, for us, weakens water type moves. Beyond that though, I would encourage you to not assume anything else. Looking at our threats list, for example, it is likely (though of course not guaranteed without that discussion happening) that Fire-type coverage would be off the table. Maybe we will have it, but maybe we won't, so if that would be a major part of why you'd want the ability, I would suggest you look elsewhere. While this is the most obvious example, this is something to remember for any ability. If we choose an ability, we will certainly make sure we get to take advantage of it in some way, but that does not mean we will necessarily use every single possible aspect of it.
With all that said, I actually do like the idea of using this ability to help us threaten water types better. Getting in, or at least not being afraid of Water types via ability is something I consider far more important for the base form, which will, by definition, not have the same sheer stats to rely on as the mega evolution.
In this regard, I like Water Absorb, as it can let us come in on those water types without a lot of fear, but of course does nothing to help us against things like the Psychic moves of Starmie or Slowbro. Furthermore, when compared to the other Water immune abilities, I think Water Absorb is the best option, as healing is something I consider incredibly useful for a Pokemon that will want to be coming in on hard hits. Even if we resist them repeated Brave Birds, for instance, can easily wear us down, and it is always nice to be able to eliminate entry hazard damage. Furthermore, when compared to Dry Skin, Water Absorb does not make us more vulnerable to fire, nor is limiting of the kinds of team archetypes it can be used with. Oh, and for what its worth, I do not consider Water immune abilities to inherently be anti-concept, especially if they are only on one form, since Water is not our most defining weakness (that would be Ground), and we still will likely have to deal with that weakness in one form.
I also kinda like Drought as an ability, as it really can make us threaten water types, by pseudo-removing our weakness to water, while also being able to, at least once, pass that resistance over to the mega form. It does of course have some limiting effects on the kinds of teams that would want to use this Pokemon, but when you have such a massively team defining ability as this, I don't think that is a problem.
Sand Stream is not a bad ability here, by any means, but honestly, it is probably my least favorite, because it is most general, and does the least to specifically help us with the water types that I think we want to deal with here. The only thing that really makes me consider this as a good possibility over the other options here would be the fact that it helps us better deal with things Manaphy that have Psychic, as if we have any other ability Psychic will hit us harder than anything it could do to us in Sand. This does, of course, also make it so Slowbro and the like also have a harder time, but with the extra power of STAB, and our ability to tailor our stats however we want, I don't think that is a major issue.
Finally, I do again want to say that I would really like people to talk more about what they actually want out of the primary base form ability. Maybe people agree with me that dealing somewhat with water types is ideal, but maybe they do not, and I would really rather people start off by talking about what they are trying to accomplish with an ability, rather than simply talking about a specific ability and why it makes us more better. The intent is important, so don't forget that.