Batboard

I bought a parts Saber I on eBay a while ago that says SUBMERSIBLE on the top deck where a secure Saber says Securenet. Was there such a thing, or is this the chassis from a Saber/R in a standard Saber case?

That is a very popular option on standard SABER-1s of that vintage, we had hundreds of them in service. The SABER-R was intended for the military market. http://batboard.batlabs.com/viewtopic.php?f=1&t=73976

Yep... that's the only truly submersible Saber ever made (assuming you keep the pressure tests up, maintain the seals, etc.) The R-series radios are IP67-type submersion... a few feet for 30 minutes. The true Submersible Sabers were good for substantial depth - I believe their first customer was actually the Navy.

Non R (military radio) submersible radio did exist for the firefighters market they were mainly model IE 2 zones 24 channels version.
You need to have, to make a submersible radio a, submersible housing it can be identified two ways:
1 - With the model number on the housing you have to remove the name tag (sticker from the front top housing and look for part number 15E05106U
2-If you remove the speaker grill (glued it can break) in front the speaker would not be a black cloth but a plastic like saran wrap to prevent water to get in.

Lots of those casing part number are available on eBay ...
batboard tread : http://batboard.batlabs.com/viewtopic.php?f=1&t=73976
Good saber info page: http://www.cryptomuseum.com/crypto/motorola/saber/

cheers !