I'd suggest Water/Bug if Stratagem wasn't so high :/ It might just be the hype, but who knows? It's still better to work with the whole top 10, though. We just need to pay more attention to the upper half of the list.
So, for now, we have a typing that makes it resist Syclant's Ice Beams... and makes it weak to Zapdos' Thunderbolt and Stratagem's Giga Drain. Great. However, it lols at the lower end of the top 10 (except for Pyroak's Grass attacks), so that's a good thing.
Offensively, it's already well-served, as Stratagem, Tyranitar and Fidgit won't like STAB Surfs/Waterfalls/Aqua Tails. Zapdos may not like Ice Beams/Punches, but we want to stop the bird also, we need to either make it speedy, or give it neutrality to Electric, that is, Water/Electric, Water/Ground (AGAIN?), Water/Dragon, or Water/Grass. None of them seems like a good idea to me, because now you're stopping Zapdos, but opening your guard to Stratagem+Tyranitar (Electric), Stratagem+Pyroak (Ground), Syclant (Dragon, but I should run some calcs beforehand), and Heatran+Syclant+Pyroak (Water/Grass). I really think it's better to let it handle Zapdos otherwise (like, not insta-dying), and worry about the others.
That is, those who will fuck up with little Watermon easily. They're Pyroak, Stratagem, Syclant, maybe Revenankh, and maybe Blissey. While Stratagem dies to Watermon's Surf, it has Tech Giga Drain, which is not a good thing; Syclant has +2 Bug Buzzes, Revenankh has those shitty Brick Breaks and tries to be immortal, and Pyroak and Blissey are two bitches. We need a type that makes Watermon not a "too much centralizing force" and also lets it check these buggers. Something that doesn't ruin it if it gets a defensive spread, or doesn't make it have a shitty type coverage, should it ever get an offensive spread.
So, with this in mind, I'm trying to come up with the best typing for it, and a really interesting option that has come to me is Water/Fire. Yes, Fire. 4x resist to Fire Blast, Ice Beam and Bullet Punch, while also getting neutrality to Giga Drain and resistance to Bug Buzz. HOWEVER, it doesn't stop Revenankh, still has Zapdos to worry of (not that it's a bad thing, I think), and more importantly, gets a weakness to Rock AND Ground. That is, those two pokémon you could laugh at to an extent (Tyranitar and Stratagem) now have a way to fuck with you (specially the latter, being blazin' fast and all). You also have to fear random Earth Powers, and more importantly, Stealth Rock. The way I see it, this poor man would either have to be FAST, have Levitate/Mountaineer, or both (and I don't think we want poll-jumping, right).
Being Fire, it becomes a gamble pokémon: You have infinite chances of switching in, have infinite ways of killing the opponent, but may also get killed by everything. Your STABs kill 6 of the top 10; 3 from the ones you kill may also kill you (Tyranitar, Stratagem, Figdit with Earthquake); you have problems with other 2 from the top 10 (Zapdos, Revenankh), and the final 2 may vary from "counters" to "easy kills" depending on how this pokémon is built.
Another interesting typing is Water/Poison. It's a nice option for those who doesn't like SR weakness, while also getting a Fight resist (hey there Revvy). It's also neutral to Grass and Rock while keeping resistance to Steel, Bug and Fire, which surely is something.
The first problem I see is that it only kills 4 out the top 10 pokémon... but hey, at least it's checking Revenankh and others and it's not meant to kill everything using only STABs. However, its main problem is that shitty weakness to Earthquake. If you wanna fight Heatran, Syclant, Stratagem and Tyranitar, you better check if they have Ground moves or not OR be Scarfed, otherwise it's going to hurt. I think we could make it to take at least (most?) one Earth Power from huh Life Orb Heatran, so it can still work as a kind of check, right..?
Trying to resume this little wall of text: If we want a offensive pokémon, we should go with Water/Fire, kill everything while trying to wall shit, without being a Strata-like glass cannon. If we want a defensive pokémon, let's go with Water/Poison and eat any attack with no "Earth" on its name.