Why would this be an Island? Wouldn't it always be better to tap for than just ?

Being an island also lets you fetch it a lot more easily. Apart from that; yeah - no one will use the : ability now manaburn is gone.

Of note: I don't think the "Opponents may steal this mana" clause works right now; everyone will always just try to drain it of mana as soon as it untaps, meaning no one ever gets it at a useful time.

I think this is trying to be Rainbow Vale. Do you want something like ": Add to your mana pool. Any player may activate this ability but only on his or her turn"?

The first line of text doesn't apply to your opponents. They can tap your Desert Pool during your upkeep to deny you the mana. You probably want to rewrite it as "Players can only activate ~ during their turn". Though, I think Alex's simple version works better.
Oh, also, if you keep Alex's version, you need to remove the "Island" subtype, since then you could tap it for blue mana during your opponent's upkeep.

Don't think you need the last sentence now; the firs tone should do it.

"as well as during your own" would actually untap it twice, since it already untaps during your untap step. You could (use reminder text) if you want to, but I think most players would get it... players, when they aren't certain, have a tendency to do things that benefit themselves anyways.

Or you could simplify it by making universal: "~ untaps during every untap step."

@jmg: I don't think your suggested problem is actually the case. Compare with the wording of Seedborn Muse and Quest for Renewal. They don't say "as well as your own", but I wouldn't expect "as well as" to alter the effect.

And TBH, even if it did somehow make your card untap twice, it wouldn't matter at all. Nobody gets priority during the untap step, and a card doesn't count as "becoming untapped" if it was already untapped, so it'd be completely identical to untapping once.

Oh, heh, yes. I don't know why I assumed that the card was untapping at beginning of upkeep, but I was. Foolish me.

I guess the only remaining problem is that an opponent could tap it for mana during their upkeep, then it untaps and they can tap it for mana. But since I don't think you're allowed to tap stuff during upkeep, it's probably fine. (And even if you can, what would you use the mana on? And as long as you remember to tap it in your endstep it can't happen...)

Er... V, did you forget the "Untap, upkeep, draw" mantra? Untap is the absolute first thing that happens in a turn, and nobody can do anything during it. If they tap this in their upkeep, they're welcome to, but it'll be tapped for their whole turn.

I meant "during your untap", so yeah, agree.

You say "You want ice cream cone?". We both say "Yes." We are twins, that is why.

You say "You want ice cream cone?". We both say "Yes." We are twins, that is why.