Clearly, installing a spa on a raised wooden deck will require substantial reinforcement. This means you will need to wire the spa to your circuit box. If you haven't wired circuits in your circuit box before, don't start now. Hire a contractor. This is important.
Electricity is dangerous.
Doing this poorly could cause injury, damage other equipment in your house, and start a fire. If it does start a fire, your insurance company will not pay your claim. So, again, if you can't do at least as good a job as a professional, hire one. The cut-off box must not be locked when the spa is in use. This won't cost you any extra unless your circuit box is full.