Another thing that can make your feet fall asleep is your heels pressing down too hard on the bottom of the boat. Sometimes more cushy footwear will help, or try a small piece of foam under your heel.
Also, lots of time your feet fall asleep while you are just floating around. Actually playing around seems to keep the blood flowing, while lounging in an eddy or on the calm water can put your feet right to sleep.
Getting flexible and used to your boat also helps. If your feet are only going to sleep, but are not in pain (rubbing or cramped) then you will eventially be fine. Time in your boat will get you more comfortable with it as your flexiblity increases. 30 minutes in a new boat before your feet go to sleep seems decent to me – it will get better. Try increasing blood flow by wiggling your toes now and again and flexing your calves to pump the venous blood back towards your heart.
Consider some more padding on the seat, but if it is a sciatic issue, you’ll have a sore and numb butt as well. Maybe try a thin piece of foam and see what happens.