Which islands/experiences are available cruise-only?

I'm planning a 3–4 week trip to the Galápagos this August and am hoping to get some advice from people who know the islands well.

My current plan is to do a combination of land-based travel and a cruise. Since I'll have plenty of time, I'm trying to be strategic about which cruise itinerary to choose. I'm especially interested in snorkeling and wildlife viewing rather than diving.

My question: **Which islands, visitor sites, or wildlife experiences are truly cruise-dependent and impossible to access via day tours from the inhabited islands?**

In other words, if I can visit something reasonably well as a day trip while staying on Santa Cruz, San Cristóbal, or Isabela, I'd rather save my cruise days for places that are otherwise inaccessible.