A great view of the Mediterranean can be had at the restaurant called La Oca del Mar (Platja De La Mar Bella Espigo Vatderroba Barc, Barcelona 08035 ES Link in Spanish). It is actually on a jetty at sea near the Mar Bella beach.  You are surrounded on three sides by water and fantastic views of beach.  The food is excellent if a little pricey, although you can get a "menu" (fixed price meal) for €10 Monday to Friday at lunch times (1:30-4:00 pm).  But if you do want to splurge get a paella and a salad.  This will set you back about 6o€ (if you include a pitcher of Sangria, of course).