Don't have all 6 as Water, as there are good non-Water types that function well in the rain. However, Water does only have 2 weaknesses, so these are rather easy to compensate for - Water/Poison and Water/Flying deal with Grass, and Water/Ground deals with Electric. But it is probably better to have other good rain Pokemon alongside your Water types on the team. Pokemon like Toxicroak, Dragonite, Jirachi, and Latias (among others) all make good choices on a rain team as well, either because of their ability (e.g. Dry Skin) or their moves (Thunder, Hurricane, good Water moves).