Magandang Hapon,
Welcome to Doha first of all and i hope that you would be having good time here. You can start swimming in May because the water is still kind of cold these days, unless you can bear with that. good places for swimming can be Sealine which has a nice beach. I don't know about seas food restaurants as well as I go fishing and whatevr i catch i make at home and enjoy it. You are right, Hamour & also King Fish tastes great in addition to other different kinds of fish. There is a good Filipino restaurant in Mansoura Street and I tried before the sea food Sizzling Seafood there and it was fine. It was a mixture of fish, squid,shrimps and mixed vegetables [Paprika, Onion, Baby Corn,etc...] in hot sauce with rice. I did try other restaurants so far, but we can look for some together.

Nice to meet you and welcome again.

Best Regards