Sao Miguel island is at roughly the the same lattitude as Maryland but there are strong currents which carry air up from Africa that make the island much warmer in the summer and more temperate all year round.
While this island is beautiful and boasts some spectacular coastline - it is a volcanic island where most of the beaches are covered in black rocks of varying sizes. Needless to say a day at the beach can become hot and uncomfortable very quickly. I would recommend when selecting a hotel, to pass over those boasting beachfront property in favor of those with a pool and/or proximity to downtown.
The weather begs for swimming but I'd recommend sticking to one of the many beautiful lakes or hotel pools.