In answer to your question: You should really only focus on one element at a time, and switching is easy since you can re-arrange your points in town. If that doesn't help then I probably misunderstood what you're asking.
And in relation to water magic: Water is a really good support element, I love playing water in PvE.
Air is the useless one in PvE, it has a few support elements but in general Earth or Water are better support and spikes are pretty pointless on monsters.
There is no element that is useless in PVE.....though fire and earth are more useful in a wider variety of areas.
The nice thing is that Sup runes are cheap now....so, do what I did and build your ele to run all 4. I literally carry 4 sets of headgear and a weapon set for each element. That flexibility keeps my ele fresh - rather than just clicking nuke chains all day.
If you have to prioritize - in order of PVE usefulness - I would say Fire-Earth-Air-Water.
Best of luck!
XBL GT: Loki ISP....GW IGN: Nentslye Woodmaster, Pope Woodmaster I, Silussa Mindraper Clan Leader, XBL Clan Wolfpacklan...www.wolfpacklan.org