I really think that the way to appropriately use the strengths of this typing is to somehow craft a Fire-type that can take on Water-types. Not only is this an unfilled niche right now (which is really practical), but it also helps a lot more than you might expect. Bulky Water-types are defined by strong neutral Water-type STAB, support Ice Beam for Dragons, and then Toxic. We already handle Ice coverage by resisting it, and we're immune to Toxic by typing! That means that if we continue this trend and plan on dealing with the Water-types, we can give this Pokemon a seriously typing-dependent niche that will be invaluable to the OU metagame. There is no better solution, if you ask me.
		
		
	 
Dusk's proposal is extremely welcomed, in my opinion. 
This not only allows it to fill a special niche, but blocks many popular special attacking types. Resistance to ice, grass, fire, and a way to fight back against water types handily would enable it to become a sort of special sponge. Poll jumping here, but an immunity to water via Water Absorb or Storm Drain would be a huge plus against rain teams and giving this mon greater specially defensive capabilities. Speaking of rain teams, if we're talking of countering water-types somehow, rain is coming into the discussion no matter what not to mention it as a way to weaken fire attacks for other types. Of course we may choose to give it moves that fight back against water types rather than giving it immunity.
With this said, resistances give it innate bulk so giving it a role of a physically defensive tank to hold its own against physical attackers with resistances that hold off popular special attacking types would be interesting, though having CAP 3 become a special wall would optimize aforementioned resistances. It would also let it better handle electric attacks which are prevalent on water-types with the esteemed BoltBeam combo. Even Starmie would have trouble handling CAP 3 since many variants do not carry psychic moves.
Might I also add that the water types mentioned have low Def. We can capitalize on this by using the unorthodox physical poison and fire moves, which are often viewed as special attacking types. (I guess flare blitz is quite the exception.)
There is another route we could take: to completely ignore the water types and focus on the rock/ground and fighting types. If we make CAP 3 have an offensive presence, other pokemon may be afraid to come in. Further amplifying its presence are the type of moves it'll probably receive. We can have the moves provide supportive effects in case CAP cannot handle certain pokes so while they switch in, they may be statused. Though they may resist the STAB options, rock pokemon are not as common as ground who are hit neutrally by fire. The only two common rock pokemon are Ttar and Terrakion, both of which would not enjoy being burned by switching in to take the hit. I feel that this approach would rely two heavily on fire rather than both typings.
For Heatran and D-nite, I think they are worth reviewing a bit. They are part of the top 6 OU pokemon so I think being able to handle a number of them would be a huge plus. If we handle water, Politoed and Rotom-W (a bit) are cleared, Scizor is out without coverage moves to handle CAP 3 efficiently while having 4x weak to fire, and Ttar not wanting to be burned (is that good enough?). 4/6, this leaves only Heatran and D-nite who can be handled, more so for the former. Might I mention that Mold Breaker would allow CAP 3 to potentially use fire moves on Heatran, hit for regular damage against D-nite, and use a coverage ground type move against Rotom-W.
I can't help but keep brainstorming of the perfect coverage move for this CAP or said abilities attached to it. Truthfully, it's as Mars hinted and Asylum explicitly said. I think we should be able to mention such ideas while mainly focusing on the discussion. I understand that by doing the threat discussion before deciding on an ability, we can truly see what kind of ability we want for CAP 3.