Hi David r
Yes as Ivan says I think the best snorkling area is proberly to the right of Arillas around the rocks and cliffs by nudee beach which isn't that busy unless your going in middle of season? and i have just seen you are going in season!!!
Myself and Norm usually do a bit of snorkling and 25m out is deep !!! no, its quite level & i would say about 20 to 30 feet? there are some rocks ,but if you go towards the island i would think that could be good?
regards
Paul/Deb5