We probably don't want the "tap" thing, but I thought I'd try the simplest variant. This spell probably wants to do something else instead/as well.

I imagine this effect being on a cantrip, probably without the tapping.

I originally thought cantrip as well, but it seems almost too straight-forward. OK, I expanded this to be tap-anything (which generally costs with a small bonus) and "flood" (it normally costs about as sorcery speed to do "become an island" but is only worth playing if its a cantrip, but this version is weaker as flood still leaves the land all its other abilities.) so this should be competitively costed but not too strong. And "tap anything" is so much more natural than the specific "tap that land" that was here before.

But there's a lot to be said for the straight-forward cantrip version too, we may change it to that.

We may be falling into the trap of making every card competitive. I don't see why this card can't just add a Flood counter at Sorcery speed with no extra abilities. It would be a better card than Sea's Claim at that rate, and sets you up to start smashing your opponent with the sets mechanic starting on round 2.

You're right, we probably do do that. However, I think it's a fair question if the flood enabler should be competitive (to help the archetype) or not (so it's not snapped up by non-flood players).

Note that would not be quite the same as sea's claim: we decided it should probably make lands islands as well, so it doesn't do mana denial, but it better at fixing your own lands.

Edit: OTOH, no, you're right. I'm not at all sure whether a "flood targent land" will be playbale, but we should probably try that version (or possibly a cantrip) first because it's the simplest and it should be playable here if anywhere, and add stuff to it if it doesn't seem interesting enough by itself. My concern is that if you want to play this on turn one, you'll have to devote a lot of cards in your deck to spells that only do something else. If our creatures end up reading "UU. 2/2. Islandwalk. As an additional cost to discard this, discard a card." they wouldn't look as good.

The cantrip version invites comparison with Aquitect's Will and Spreading Seas. So it would probably need to be 2 mana.

I do like the principle of having each flood card at a different CMC as a first stab, although it's fine if dev deviates from that a bit.

I feel like having it tap is nice flavor.