"Rock Island" → Gálnos’chrónn
"Fish Island" → Másyr’chrónn
Chances are if these are the names of islands (place-names), they’d erode down to more unitary forms over time. Especially if the isles are inhabited by non-elvish races. Let me know if that’s the case, and we can come up with something.

For "whirlpool," I’d agree with Mina on élvi’thyrón. "Twirling water" (élvi’már) could also work. Both of these are preferable to térqu’mar’avásh for two reasons: terquán is definitely a noun describing the concept of turning (cf. other -án nouns like sae’llán "music") and so cannot be used as a verb; and the compound is unwieldy even by elvish standards, haha. They wouldn’t need such a complex compound for the idea of a whirlpool.