Wording is a bit odd. Normally these effects can be circumvented by something like Freed from the Real, but this one can't.

The wording is probably impossible, as well. I don't think you can do anything during the untap step, including activating mana sources. Most triggers during the untap step silently carry over into the beginning of the upkeep.

I feared this didn't work, but I didn't know why. Thanks.
Would "Whenever an island you control becomes tapped, tap enchanted creature" work?

Hee. Certainly. That'd be pretty unique and yet simple too.