Every (old) boat has osmosis. No boat has ever sank of osmosis. Don't worry about it. Although obviously try to buy a boat without too many big blisters as but don't worry (Unless as Vicking said and it's a new boat as it shouldnt really affect a new boat), but it's not going to affect it (apart from people worrying when you come to sell it). As always get a prepurchase survey before buying.