I have a ten year old Sweetwater Bahia hot tub. We just drained, refilled, put a new filter, new t-stat in and the water heats, but only up to 94 degrees....is my heater going bad? also, if yoku change a heater element, does the tub need to be drained?

If you can get to 94, it's probably not a heater issue. Make sure your thermostat is still calling for heat above 94, that your high limit isn't tripping, and that you have your cover on.

As for removing the water to replace an element that would depend on whether or not you have working valves at the equipment. If they are there, there will be one just in front of the pump on the suction side and then another just after the heater on the pressure side.