The 1500 watt heater will draw about 12.5 amps and the March pump is rated at 1.5 amps for a total of 14 amps, which is just under the nominal 15 amp rating of the breaker. However, you typically oversize the circuit by 20%, to account for various things including variation in the actual trip point of the breaker, so the 1500 watts for the element becomes 1750 watts or 15 amps, which about the max load you should put on the 15 amp circuit. (Ever notice that 1500 watts is the maximum wattage hairdryer that you can find). So even though the breaker might not pop with the pump is running, it should probably be on it's own circuit.
The 50' of 14 awg just adds resistance to the circuit which will reduce the amount of current flowing and therefore will reduce the likelihood of the breaker popping.
Paul