But I'm wondering why one would deploy an SMB while still underwater. Can someone please explain?
Thanks in advance.
SMBs are typically used if required by law to mark a diver's location at all times during a dive. Hence, they are deployed at the surface prior to a dive and towed about. A rescue sausage is often deployed at depth and (depending upon its design) can be orally inflated, inflated with your reg, or both.
A rescue sausage can be used as an SMB - on a dive last year a passing boat cut the line between us and our SMB - we deployed a rescue sausage and continued on with the dive, in full compliance with all local ordinances. In addition to not being fined, our boat managed to keep track of us on a drift dive in very choppy water - had they not been able to stay with us, our extraction from the water may have been delayed.
At the surface an SMB or a rescue sausage can serve a welcome extra flotation in the event that your extraction is delayed. However, if you want to be spotted in choppy water,
size does mater - get a long, bright rescue sausage.
In the (very unlikely) event that there is a catastrophic failure of your drysuit and/or buoyancy device, a rescue sausage can be rapidly deployed to "tether" you to the surface via the spool of line. Remember that a rescue sausage may have a buoyancy of about 25#, but a fit swimmer with fins will have a very difficult time raising 5# to the surface from depth. Summary: it is much easier to pull yourself up using a rescue sausage/spool than to swim up.
Final note: everyone has their own opinion on what kind of spool to use with a rescue sausage. I prefer a finger spool as, owing to its simplicity, it will not "jam." Once un-clipped, if the spool gets yanked out of your fingers it will continue to un-spool and may eventually fall past you as opposed to a cave-style reel which (if jammed) will rocket to the surface with your sausage. Finger spools are also smaller and cheaper.
Happy diving!