The data sheet document for the 400 Series states in its camera cleaning section that water should not be used on the camera lens. Also, on the D415 and D435 models, the grille on the casing would be an obvious means for water to penetrate to the inside. So using the 400 Series unprotected in the wet is likely not a good idea. Apart from that, the cameras otherwise work very well outdoors.
However, RealSense cameras have been used underwater with protective housings. Answers 3 and 4 on the discussion linked to below provide information about using RealSense underwater.
Intel sell a pre-made housing for the caseless RealSense R200 camera board in the drone section of their online store that has additional space in it for a single-board computer (intended for the Intel Aero controller board). I'm hesitant to recommend it though as the front cover plate likely would not line up with the camera lens positions on the 400 Series, even if a 400 Series board could be made to fit the housing.
If you have the ability to 3D-print, the official CAD file models for the 400 Series may be of more use to you.