Casting with a 9 ft rod out of a float tube is IMHO not necessarily a problem. Back cast up at an angle of about 45 degrees, starting low. If you cast fast the angle can be lower. Point your rod out to the side at an angle instead of over heading. Longer rod helps if tip action but it's a minimal difference compared to technique, so practice and learn to cast fast (narrow loop). You will find that after a while you will be able to cast 60, 70 ft or more. Mind wind blowing on your casting arm (especially if you are a slow caster) Try to avoid that in the beginning. A 10 ft 3 wt probably will pull your line lower than a 9 ft 5 weight due to the medium action such a rod tends to have. A tip action 10 ft 3 might be of help but it is not a rod you see every day. I am not aware of the other conditions you are fishing in, but it might be so that a 10 ft 5 weight tip-action rod will be more suited and for sure easier to lay hands on.