When I fish from my Old Town canoe I sit flat on the bottom. Due to the smooth, flat bottom (on the canoe) I found this was the most stable for me to learn and just never changed. Down that low you can easily transfer to a kayak or float tube. Learning to cast that close to the water will force you to 'keep thy back cast up' and make you a better caster even when not in a watercraft. As advised above start short and go longer as you get more comfortable.