Terminology definitely gets confusing. I don't think there is any "standard" that manufacturers abide by, either. Besides just being a sealant (which I would generally equate/interchange with caulk), you can have adhesive-sealants.... and then "how" adhesive is the sealant. Most, if not all, adhesives will ALSO seal simply due to what they are doing/what they are. But I totally agree that something like 5200 is absolutely the wrong product to use. Then there's also the question of whether something is UV stable (4200/5200 is not) and whether any of the sealant is going to see the sun (or reflected from the water).
There's obviously more than one choice here, and it really comes down to what a person's "favorite" is. For me, I've been using the BoatLife stuff for nearly 20 years and they haven't let me down. Plus, I only need to decide between two products for everything I do... LifeSeal or LifeCalk. In this particular case, since there are mechanical fasteners being use, there's no need for a strong adhesive (all sealant will be at least somewhat adhesive due to their nature) so I'd use the LifeSeal... and it saves me from having stained fingers for the next few days if I had used LifeCalk... since I almost never think to use gloves till AFTERWARD. You know, the whole "I'm not doing a whole lot of sealing here - I'll be careful - why waste the extra 30 seconds to go and get gloves" mentality.
Side note... as of the last few years, I've also started to use butyl rubber tape... which is probably the king of sealants when used between two things (bow rail stanchion plate, for example).