I know this part of Spain really well. Most people head for L'Estartit. If you fly into Gerona (;ow cost flights from Stanstead and Birmingham?) you could hire a car and go to Aiguablava, where there is a Tec diving centre (used to be owned by Barcelona Scuba Club) and an all year Parador that does deals in Winter, or to Aguafreda where the dive centre is Sarascassa. On the headland is Cap San Sal a magnificent ex-hotel now offering decent priced apartment accomodation. Spanish Harbour Holidays are the self catering specialist in this part of Spain. The sea is cool out of season - you'll need a semi dry or dry suit. The coast around Begur (as per "resorts" above) is a marine reserve.
Unfortunately the software won't let me paste links here, so try Google. Let me know how you get on!