When I see the spectrometer with its screen, I immediately have the question: When you put that into a box, you will not see the screen. So, What do you wanted to record and what do you need the fibre optic for? Maybe there are better solutions.

Anyhow, bluerobotics.com  sells a 300x190x80 mm Alubox that is rated for 300 m for US$ 830. I guess, that is a completely overkill. Not sure if there is are pelican cases that are equivalent to this. 

Hi Connie. You could check out Blue Robotics (bluerobotics.com) - they seem to have some options for underwater housings and cable connectors. I haven’t personally tried any, but they might be worth a look. Good luck with your project!

If you don't mind sacrificing the fiber optic cable, you could pretty much take any submersible housing (rubber coat 3D print or alike) and utilize a potted penetrator. Using something like 3M™ Scotchcast™ Potting Kit 2136 or a simple marine grade epoxy from the hardware store should work. You could even put a polycarbonate window to see inside the housing. But this is all relative to how long you want to submerge the instrument for and how reliable you want the housing to work. 

For a cable like that, I would drill a hole into your underwater housing a little larger than your cable. Take a 2cm PVC pipe which is a bit larger than your hole and hotglue it onto the housing making a seal for you to fill with epoxy. Place another 1cm PVC pipe inside the housing on the other side of your hole inside the housing. Place the cable through and pour in the epoxy. You may have to force it into the PVC with some Q tips and make sure to get all the bubbles out. A simple trick is to put it under a vacuum if you are able or a less effective option is to place it on a shaker table for a few minutes. It can be a little messy. Note: the key for any penetrator is to not place strain on your cables. 

Otherwise, I would suggest looking at Sexton Underwater Housing Corporation if you're looking for something custom that fits your needs. We have a few fiber optic connections on our things from them. The underwater box from bluerobotics may be helpful, but you do run into your penetrator issue and you can't see into the box. Other note when housing expensive equipment: ALWAYS have a place to vacuum test your underwater housing. This will ensure you don't have any small leaks before deployment. I'd skip bluerobotics vacuum handpump (I'd look around for a less expensive option bc it will break and they're cheaper elsewhere). I spent probably a couple days finding out that there wasn't a leak in the housing but the hand pump itself. You can get the vent plug and vacuum plug from blue and then just get a small electric pump (Diaphragm Vacuum Pump, Compact Oil-free Vacuum Pump Pressure Pump) with a gauge to save your hands and time.