never been to Benedorm m8 so I can't tell you specifics, I can only give you the benefit of my experinces so far fishing around the Med (Spain, Portugal, Italy, Greece, Tunisia etc)
The Marinas are generally full of fish (mostly big fat Mullet) which will happily feed in shoals on whatever you throw them (bread is best) but unfortunately you aren't allowed to fish the Marinas.
The Sea fishing from the shore is generally quite poor unless you hit on a particularly good place (look for deep water from rocks/jetties etc and use groundbait if you can) On the upside you can fish extreemly light, I generally use my heavy quiver tip rod from the beach but I've only ever caught the odd fish usually to fish baits on smallish hooks (say size 10 or so) though you can usually buy marine worms from local tackle shops (there's usually one somewhere)
If you want some serious sea fishing book in on a boat trip, they aren't cheap (30 Euros plus for a 4 hour trip, half for observers) though they provide everything you need including bait (usually cockles and squid)
The coarse fishing in Southern Spain is terrific (my personal opinion) because for the mosty part it is free or very cheap and you can expect good catches if you're prepared to experiment. You are supposed to buy a licence for the class of water you want to fish (price varies accordingly) but very few people seem to bother. I believe you can get them from Post Offices. Southern Spain has major problems with drought conditions (last year was the worst for 120 years) consequently there are reserviours absolutely everywhere in the mountains which are often only a taxi ride from the coast. As for the species there I can't help you much never having fished them but most are comercial fisheries with reasonably priced day tickets.
My main interest has been in fishing rivers where they meet the sea. Because there is no tide to speak of on the Med you can catch freshwater species within yards of where the river meets the sea. Expect loads of Mullet, Eels, Carp, Barbel and all sorts of stuff you wont recognise. If you read my previous post you'll see what methods I used, the best general advice I can offer is keep it light and use bomb-proof bait (sweetcorn is too soft)
One final bit of advice, take what you can with you. Not only will you save time and money but I've rarely found coarse gear on sale in tackle shops in Spain and you might find it difficult to get things you want (I've never seen maggotts for sale) This last time I took everything I could including 5kg of bread/hemp groundbait. I should have taken more hooks than I did, particularly in the size 10 to 16 range.
Hope I've been of help m8, have a good search on Google, there's quite a lot of stuff about fishing in Spain. Feel free to email me if you need anything else. Kindest regards, Phil