Well, I use an all water team in Emerald, but I usually give at least three of my water POKéMON, Ice Beam. Use pure water, but stick on "Ice Beam" to cover all of those overused Dragon POKéMON.
Water/Ice is vunerable to quite a lot of moves, whereas Water types are only weak to two types. Still, that's not to say all Water/Ice are automatically bad (I wouldn't call Lapras and the like bad anyway), I just prefer Water types, although not the generic Pokedex filler ones.
I prefer water pokemon because they don't have the Rock/Fighting weaknesses and fire does normal damage against them. Also, water pokemon learn Ice attacks so why have the extra weaknesses without much benefit...I do like Dewgonog though /_\
Pure Water is almost always the better choice. The only real drawbacks are it doesn't gain a type power boost and you have to use something like TMs or Move Tutors to get Ice moves.
Water/Ice has a drawback when it doesn't get as much of a type boost power up with either type then a pure type. Not to mention it has cheap weaknesses to Rock and Fighting.
Water because the Ice trait gives it more weaknesses. They can dish out more damage with their Water-type attacks and normally have a high Sp. Atk stat which can make their Ice-type attacks stronger at the same time.