'at sea' and 'at the sea' is the same I guess. But I would use 'at the sea' when the 'sea' was mentioned previously. And 'at sea' with the word 'out' infront e.g.: I am at the sea now. I am out at sea. (Not too sure, sorry!) 'On the sea' would mean something is on it : -A ball is floating on the sea - There is a boat on the sea 'In the sea' can mean inside the sea itself: - There are many fishes in the sea or it could mean the AREA the sea is: 'There are many boats in the sea' 1. I would say "In" is the right one because you are talking about something happening IN Boracay. 2. kite-surfing on the sea. (Basically, you are on the sea, not inside under the water) 3. Correct 4. 'At the beach' would mean you are at that place, whichever parts of the beach, you are at that AREA. ("on the beach" is not wrong but "at the beach" is more preferred to "on the beach" to me) Not the best answers but I hoped it helped! ^^

To add to what Pocket says: I wouldn't say "at the sea". Either "at sea" of you are on a boat or "at the seaside" if you are on land by the sea. "On the beach" means on the sand, "at the beach" is in the area of the beach, maybe on the sand or maybe not.

@amul, thanks

'At sea' has two meanings: 1. The ship is at sea (ie the ship is sailing on the sea). 2. I am all at sea with this problem (I am totally confused and have no idea how to solve this problem) I can't see where I would use 'at the sea'. Regarding your questions, one normally talks about being ON an island not IN or AT. Kite-surfing on the sea.