If it is just for a hobbie to explore - I would suggest putting your money on tether length and lights rather than survey-grade ROV. You might be hard-pressed to find a decent ROV in the <$3000 range.
The expected water clarity and associated light penetration at the locations/depths you wish to survey are really the main drivers of choice for success. FW is generally clearer than SW - on average. Exceptions are during floods where run-off turbidity and/or bubbles entrained in the flow at the surface makes it tough. The worst conditions are during spring bloom in the salt chuck where you water clarity might only be 2-3 feet. During those times and camera/ROV will only be a few feet off the bottom to see the bottom - and you will need to be quick on adjusting your tether to suit conditions such as bottom unevenness, rocks and drifting by your boat.
And as with most things in life - going deeper costs you more - and it is darker and the red spectrum is absorbed in the 1st 30ft or so (dependent upon water clarity). Extra lights are normally required after that depth.
There are a number of readily-available sportie tethered cameras models on amazon or sportsfishing online sales. Most production models & sales focus on only a 50ft tether and come with infrared lights (like the Aqua View series that RG mentions). The cheapest models with only a 50ft tether start at ~$300-500 but you can't record anything from them. No feeds nor SD card slots. They are good for the shallower depths and for a quick look only.
I would look at getting the model with longest tether you can order (likely 100m or 300ft). Those models start at $600 and go up to $2000, usually.
In shallower areas with adequate light - in a pinch one can use a GoPro but you really have no idea what it is recording or if it is recording until you retrieve it. I use a GoPro on an extendable gutter pole to record spawning salmon but in only up to like 10ft depths.