Hi there,
yes it's possible however the snorkeling might be compromised a bit.
The diving locations at the Marietas Islands are by the island n1, snorkeling instead can happen by both islands.
If there is a diver on the boat the boat has to be the whole time close to the diver and, because the national park rules do not allow to drop people and leave, this means the snorkelers too will need to be by the boat the whole time.
If we go out on a snorkeling trip only we choose the best snorkeling location according the conditions of the day, we might stop by the first island or by the second one.
If there is a diver on the boat you will need to stay by the first island no matter the conditions.
We can take you to check out the beach at the second island in between the 2 dives but that will be a quick stop.
So yes we can do it but that all depends on how flexible the snorkelers are.
Another option that would give us more flexibility and would work best for diver and snorkelers is to have the diver going for 1 dive only instead of 2.
Let me know if you think this can work out and I will send you more detailed info over.
Roberta