do i need a fishing licience to fish in the sea at the port in salou

According to this site:
http://www.spain.info/reportajes/espana_aguas_perfectas_para_la_pesca_recreativa.html?l=es
yes, you do need a licence. I am not sure if it is strictly enforced, but for peace of mind I would enquire at the bait and tackle shop you visit as to whether it is strictly neccessary. I have never seen anybody questioning people fishing from the beach being asked to produce a permit...but that doesn't mean it doesn't happen. I doubt it is at all expensive, and I would recommend you buy one if it is required, for your peace of mind. Meanwhile, I am sure somebody will be along soon to confirm or deny it categorically

Officially YES you do need a licence.
They can only be acquired from the Ministery Of Agriculture in Tarragona with a photocopy of your passport and a credit card, in person.
OR go to www.telelicencia.com and you can arrange it online.

Or you can go to the fishing tackle shop near the train lines on calle Barcelona, and he´ll do it for you, need N.I.E, or passport.Renewed mine last year about twenty odd euros for 2 years, but you can get an annual licence or indeed a 5 year one.You pays your money, and he´ll have it done for you the next day.
Dont think theyd bother anyone on the rocks in the hight of summer, licence mainly for fishing the Ebro, or other rivers, where they are more strict and have the powers to confiscate your tackle.
By the letter of the law here, you should also have public liability insurance when fishing, in case you hook a passing tourist, or a 6 oz lead goes through someones car windscreen.
In brief (lol), youd be very unfortunate to get busted tiddler bashing on the rocks in Salou without a licence.

P.S. Try using a ball of mushed up white bread, and dairylea triangles as bait. fished about 2/3 feet deep just off the rocks, get a bite a chuck, tiddlers mainly but the odd decent Dorada. Enjoy, and dont forget, its not the kill, its the thrill of the chase.