The box you are looking at is set up for 220vac. This circuit uses a 2 pole breaker at the breaker box. If youlook at or inside the box you will see that there is single pole breakers and double pole breakers. Single pole are 120vac 2 pole are 220vac. Change out the breaker with a single pole breaker. This will give you your 115vac. DO NOT use the 2 pole with only 1 wir going to it!!!!
Consult electrician for proper breaker size for the wire that is run.

Check the wiring, its voltage, an electrician, and inspector before doing anything. Many Jacuzzi's were wired without the necessary white/neutral/grounded conductor that is need for 120v because they just ran 2 black/hot conductors with each one carrying 120v. If they wired it for 120, then they would have 2 hot wires(both black or black & red), the white/neutral, and then a bare or green ground wire. If you have a total of 4 wires, then one hot wire would remain capped and you have 120v after changing the breaker.

If you only have 2 hots and a ground, then you may get by (if your local code inspector oks it) by wrapping white tape or painting the black wire to be used as a neutral on both ends. The black would go into your new single pole breaker and the white would go on the neutral bar. Also use a weatherproof outlet.