Moab's tough from shore - you'll need to rent a boat. But you can row from one end to the other in 20 minutes, so a motor and battery isn't needed.
This time of year, try to be on the water in the early morning or later in the afternoon/evening - and if you're fly fishing, use dark coloured weighted nymphs (no lead in the park though).
The hike in is short - fine for 5 year old. I wheel my 12 ft kayak in there no problem.
Here's some shots at Moab from this spring...
I'm not sure about the whitefish on the Snaring or Athabasca, but the ones on the Bow in Canmore haven't started schooling up much yet. Give them a couple more weeks before that fishery heats up.