Actually, I put in 230'. Over 250' is pushing it. Don't forget to add vertical distance also. Keep it under 5.5%. I'm sure the heater won't care but it will cut down on the efficiency.
Wouldn't be a bad idea to put a little sub panel out there as Brett said. Run a 40A/2P 240V #8s to a little 4 space panel , then you could branch off with 20A, #12 120V circuits no problem. Its probably a wash in cost, those little panels are $30 (well they were last I knew).