If you are in lock down you can still use your garden and you can still use your hot tub. Ensure you only use it with people you are living with and that they show no symptoms of coronavirus.
Pool, Hot Tub, & Sauna Safety During COVID-19
According to CDC – ‘There is no evidence that COVID-19 can be spread to humans through the use of pools and hot tubs. Proper operation, maintenance, and disinfection (e.g., with chlorine and bromine) of pools and hot tubs should remove or inactivate the virus that causes COVID-19.’
That being said, the close proximity to others in the hot tub may put you at risk, if they show no symptoms but have the virus, this maybe passed in droplets in the air when speaking for example.
While hot tubs and pools are safe to use, just be mindful of the proximity to others and hygiene.