For those wishing to get fancy .... it's quite reasonable to get significant heat from the sun. That is, you pump the water through a coil of 5/8 irrigation tubing atop the gazebo, enclosed in a safety glass faced case. I've seen these coils used to heat full size pools; they should be even more effective with a smaller tub.
The fun would come in designing the controls
The controls are not really that tricky but you will need a valve actuator and they can get pricey.
Then all you really need is a thermostat that picks a relay controlling the actuator that directs water to your solar collector.
I am doing something similar with my pool, no fancy electronics, just a few relays and some switches.
I got real lucky and found a guy selling some Pentair valve actuators cheap on Ebay.
The real trick is keeping the heat in the spa. You need to insulate it and keep it covered, whether you solar heat it or pay the PoCo.
