Max Pressure or Depth? Required Data rate? Acoustic or RF? Passive or active? Distance to receiver? Cost range? Single or multiple use?
PM if you can’t answer here.

Look at Omega products .
They are a bit expensive, but work well.

If wireless is not a requirement it would be a lot easier to just make something to do it, and it would be way cheaper too.

You would need either a thermistor or a Dallas instruments temp sensor (your choice), a small micro, attiny85 or 84, and an 8×2 character LCD.

I made a handheld temperature probe with similar parts (just used something more overkill than a single microcontroller) since I had no thermocouple for my multimeter, it was dead simple to make, and is still handy. I could even get it to do datalogging with a couple parts and some software changes.

If something similar to that would work, I could even make it bare bones without a case, I could build one up to whatever needs you have, list it on ebay, and then give you the link and just mail it off to you.

Wireless is harder to do DIY, you need the transmitter and battery as well which increase the size, especially considering how long you want it to work for. With wired you just need a couple wires heading underwater and have the device battery or wall wart powered.

I’ve had conversations with @XOIIO before and am confident he can do it.
It is important to know the answers to the questions I posed.

Yes they do, your system will depend on many factors but expect to do some level of system integration. I did this for a living for almost decade, we even had them transmitting data over the idriduim satellites.

Onset computer corp is your resource if you don’t need wireless.

Do you have any aquarium stores in your area? You can find a ton of nice thermometers that are both small and highly accurate(dependent on price). Some are wireless others aren’t. Here’s a site to give you some ideas.