i don't know about sosua but here in POP you can fish off the coast, so to speak. local fishermen walk far into the ocean (reef) and fish from there. other go spear fishing.
i think you could maybe skip the tourist thingy and have locals guide you: will cost pennies and you may have good time.
one day you need to come to maimon (a small village close to POP): this is a seafood mecca for dominicans. various restaurants line the main street. there are all kinds of fish, langosta, shrimps, crabs, octopus, calamari... dominican food at dominican prices.
we had our charcoal grill hand made from stainless steel beer barrel. it even has a chimney. works great too. we use something called VOLCANO as a lighter liquid, sold in all supermarkets.
you can also use brugal 151