I downloaded the trial of ReefMaster and tried to view .sl3 and .smf.

It crashed ReefMaster when I tried to play either file.

I downloaded ReefMaster 2 and tried both files, again.

I get Database Error! File is encrypted or is not a database.

Any ideas?

It is a structure scan

It sounds like you are using ReefMaster and trying to open a .sl3 file when the application first starts up. You need to open (or create) a workspace first, and then you can import the sl3 file into the workspace so that you can view the track and sonar data. SMF structure scan files are not supported, but the same data can be read from the sonar log file. Take a look at the manual (https://reefmaster.com.au/reference2/index.htm), in particular the pages on the workspace and importing GPS assets.