I am planning to run underground wiring for a new hot tub. The wiring required is awg 6/3 for a 50 amp breaker. The wiring run will be approximately 110 feet.
Question: If I use underground cable, do I need to encase it in electrical PVC? Also, how deep do I need to go underground with the cable?

Personally I would run 2 #6's, a number 8, and a 10 for ground in PVC. We can't bury cable down here, I would put 1"PVC instead of 3/4" PVC so you could pull 120 power out there, maybe travellers to allow switching on and off lights from both locations.

You CANNOT run cable for the outdoor portion to a spa or hot tub. You MUST use conductors in conduit with an insulated ground.

I am of the STRONG opinion that wiring a spa is NOT a DIY job. Leave something like this to a pro.

It will also need GFI protection and Disconnect with in reach/sight, different codes for different areas.