The most important thing to consider when photographing long exposures is control. You need to have complete, manual control of your camera settings, particularly the shutter speed, and you also need the ability to set your camera to Bulb mode (also called Time mode on some cameras). Most DSLRs and mirrorless camera bodies will accommodate this, and the particular brand you choose is not important. As long as it allows you to manually control your camera and create exposures that last longer than 30-seconds, you have a camera that will work well for long exposures.
Here are some other things to consider when choosing a camera for long-exposure water photography:
Full-frame vs. Crop-frame
There is very little difference in terms of the quality of your image or requirements for creating a long exposure when using full-frame versus a crop-frame camera. The only consideration would be focal length, because you may find certain locations work well with a wider lens. A crop-sensor lens, by design, will increase the perceived focal length of a given lens. For example, a 24mm lens would appear to be cropped to about the equivalent of a 38mm lens on a crop-sensor camera.
Compatibility with Cable Releases
Its essential that you have a cable release that allows you to hold the shutter down and lock it for long exposures. Almost all wired cable releases will do this, so if thats what you use then you’ll be all set. Any cable release is fine; it doesn’t need to be high-tech or fancy. When using Canon cameras, I use the basic Canon Remote Switch RS-80N3. If you use a wireless release for your camera and want to create very long exposures, you may need to consult the user manual to make sure that you can hold the shutter open for longer than 30 seconds.
While a fixed-lens camera may be easy to tote around, not having the ability to change lenses could mean you lose the advantage of drastically altering or improving your composition. A camera that allows for interchangeable lenses is not a requirement, but its definitely worth considering in order to open up your options for framing your shots.
To achieve a very long exposure in normal weather conditions, adding filters to your lens is a must. Most, if not all, lenses for DSLRs and mirrorless camera will have this capability, but I thought it worth mentioning in case you want to use a non-standard camera or lens.