of course you can swim in May in the sea, but not "around" the island of Sal.
Only in the south there are really beautiful beaches. To go into the water depends on the waves and the wind. Be careful, if the wind comes from inland (seaward wind)!!
Sometimes there are flags (red, yellow or green) at the beaches.
If there is shown the red one, don't dare to go into the water. It is more dangerous as it seems to be.
Yellow means, be careful and don't swim too fare from the coast.
Green means, it is all right, but nevertheless beware of higher waves.