I think that the general lack of response to your question is because you're asking us to predict the weather.
I don't know if it's worth it or not but if you want to be sure that the pool is good and warm then you should go ahead and make the purchase then use it if you need it.
Average monthly temperatures for the whole month of April, according to the Weather Channel, are an average high of 83F and an average low of 60F. I suppose that means that the average temps for earlier in the month are lower than those figures and later in the month they're higher.
I'd suggest that you find out from the owner or management company if you can either wait to make a decision until closer to your vacation then you could at least add the benefit of more accurate forecasting to your decision-making, or ask if you could pay for it now and get back to the owner/management company with a final decision later with the expectation that you'd get your money back if you decide not to use the pool heater after all.
Have a great April vacation!
Dick Taylor