I really like the "Water-Immunity" abilities. I can see all of them working really well for us if we want to be able to capitalize on free switches, which Diamond Storm lends itself to with a chance to get free buffs. These abilities also distinguish us from the many prominent Ground-types in the tier, and give us a very solid use case even without considering DS. They all have up and downs though, which I've listed here for posterity.
Dry Skin: This one is kinda bad. We want to beat Fire-types over the head with our powerful moves, and adding a "weakness" to a type we're supposed to be powerful against doesn't seem like a good idea, especially in a metagame with strong Fire STAB flying around from Astrolotl, Heatran, and Blacephalon.
Water Absorb: The vanilla option is probably the best one for us here. Restoring 1/4 HP for switching in on a predicted Water move is pretty strong. There's no downsides to this either, with the only exception that it's fairly easy to play around, especially if we don't have a second viable ability. The team utility afforded by this free immunity is also worth noting, as less useful mons with a Water weakness gain substantial buffs when the opponent can't simply throw out Hydro Pumps.
Storm Drain: This ability doesn't synergize well with Diamond Storm, but I can still see arguments for a mixed attacker with Storm Drain being a possibility. I just don't think that's the way the project has been going so far, and giving us an ability that only benefits the attacking stat opposite our unique move just seems...wrong.
Other abilities I've grown attached to are Mold Breaker, Weak Armor, Unaware, and Battle Armor.
Mold Breaker: I wouldn't give this as it's only ability, but I think there's solid arguments for why this would be a strong option. Specifically it would let us handle Levitate users like Equilibra, Stratagem, Hydreigon, and others. There's really not much else to this ability. Just gives us better matchups against specific mons we might struggle with later.
Weak Armor: Hochi Mama. This ability is funky. It has the strangest possible synergy with Diamond Storm, and is actually kinda strong if we can capitalize on it later. Speed boosts aren't anything to laugh at, and with such a strong chance to undo the defense drops passively, it looks better and better. The one thing to keep in mind is that we have to get hit to get anything out of it, and if we're offensively oriented, low HP is almost a given.
Unaware: Do you hate setup sweepers? How about bulky walls that just sit on you with passive buffs (VENOMICON)? Then this is the way to go. We can benefit from this ability both offensively and defensively. It's a rather understated ability, but it sure does what it says on the can.
Battle Armor: I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits
(But actually getting crit through our defense buffs will suck)
Dry Skin: This one is kinda bad. We want to beat Fire-types over the head with our powerful moves, and adding a "weakness" to a type we're supposed to be powerful against doesn't seem like a good idea, especially in a metagame with strong Fire STAB flying around from Astrolotl, Heatran, and Blacephalon.
Water Absorb: The vanilla option is probably the best one for us here. Restoring 1/4 HP for switching in on a predicted Water move is pretty strong. There's no downsides to this either, with the only exception that it's fairly easy to play around, especially if we don't have a second viable ability. The team utility afforded by this free immunity is also worth noting, as less useful mons with a Water weakness gain substantial buffs when the opponent can't simply throw out Hydro Pumps.
Storm Drain: This ability doesn't synergize well with Diamond Storm, but I can still see arguments for a mixed attacker with Storm Drain being a possibility. I just don't think that's the way the project has been going so far, and giving us an ability that only benefits the attacking stat opposite our unique move just seems...wrong.
Other abilities I've grown attached to are Mold Breaker, Weak Armor, Unaware, and Battle Armor.
Mold Breaker: I wouldn't give this as it's only ability, but I think there's solid arguments for why this would be a strong option. Specifically it would let us handle Levitate users like Equilibra, Stratagem, Hydreigon, and others. There's really not much else to this ability. Just gives us better matchups against specific mons we might struggle with later.
Weak Armor: Hochi Mama. This ability is funky. It has the strangest possible synergy with Diamond Storm, and is actually kinda strong if we can capitalize on it later. Speed boosts aren't anything to laugh at, and with such a strong chance to undo the defense drops passively, it looks better and better. The one thing to keep in mind is that we have to get hit to get anything out of it, and if we're offensively oriented, low HP is almost a given.
Unaware: Do you hate setup sweepers? How about bulky walls that just sit on you with passive buffs (VENOMICON)? Then this is the way to go. We can benefit from this ability both offensively and defensively. It's a rather understated ability, but it sure does what it says on the can.
Battle Armor: I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits I hate dying to crits
(But actually getting crit through our defense buffs will suck)