SLR housing with zoom lens
For your kind of application, we would recommend one of the following ewa-marine housings, depending on the size camera body you have and intend to use:
|for large SLR cameras||Canon 1D or 5D, 80D
Nikon D510 or D5
|for smaller SLR and
large mirrorless cameras
|Canon 750D, Canon Rebel range
Sony A7 (all versions)
Fuji X-T1 and X-T2
If you want to use multiple bodies in the housing, please keep in mind, that you can always use a smaller boddy in a larger housing. But using a large body in a housing that we consider too small will likely lead to the housing failing some time in the future, even if it seems to work, with a squeeze, the first few times.
Using a smaller camera in a larger housing poses no risks, but will defenitely decrease ease of handling and ergonomics a bit.