First, yes...Many have waterproofed the Sovereign in a water tight box or even a water tight marine bag. Just search for "waterproof" or "waterproofing" in this forum and also in the beach forum and you'll see lots of ideas and projects done with it. The FBS machines won't do any better than the BBS Sovereign or Excal in terms of beach or water hunting. Some even say the BBS machines hit harder on gold rings at depth.
For trashy parks that's were the Sovereign excels. Sure, it's slow like any Minelab but if you sweep slow recovery speed to unmask stuff isn't an issue. It will also find coins in iron better than any machine I've ever owned. The notch doesn't kill everything below it. If I remember right in my tests it's about 12.5 digits wide on the VDI. By raising it just high enough to kill a 165 VDI # pulltab you'll elimate roughly 85% of all tabs (round or square) and still be able to recover the vast majority of rings. We scanned in over 100 rings and a bunch of tabs and ran those numbers. Of course you don't want to do this at the beach, but ring hunting in a park filled with tabs this strategy has some merit. Also, most rings will sound round, warm, and smooth, and lock into one or maybe two VDI numbers. If they sound hollow, bangy, harsh, or change VDI numbers by 3 digits or more then chances are it's trash. Trash not uniform in shape will change it's VDI a lot based on which way you sweep over it.
The Sovereign is also as deep as any machine on the market in terms of hunting for old coins, so you've got your bases covered there too.
For water hunting I just put mine in the Minelab hip/chest mount bag and throw that over my shoulder and around my left arm so it's roughly up by my left shoulder. Before putting it in the bag I put a plastic bag around it and seal it off with rubber bands at the coil cable (hole poked through the bag for this), then ziplock the top of the bag after adjusting my controls. The headphone plug I just poke through the plastic into the machine. With it sealed up this way I'm confident I can withstand the rogue wave or something that hits it without water getting to the machine. I wade up to my belly button without issues, but of course you can't do this with bad waves.
The tone ID is excellent and many guys just hunt by using that. It has the best audio in both tone and also target traits than any machine I've owned. However, a meter will for sure give you even more ability to split hairs on targets. The Sovereign has the highest resolution from foil all the way up to copper penny on any machine I've ever used. It makes it real easy to find nickles while avoiding tabs and other junk, and also to pick out a few pesky tab numbers to note and avoid while digging all other signals around it for a gold ring.
Coins from copper pennies on up get compressed into the 180 number, but for old coin hunting that's all you need. I've dug plenty of silver dimes that read like other coins so if it's deep or in trash I don't trust having resolution on a machine to tell me what kind of coin it thinks it is. I just want to know it's a coin and dig. The Sovereign can ID zinc pennies though with a lower number, as well as some other coins such as indians. For clad hunting the only coin I want to be able to ID and avoid is zincs so that's all the ability I want for that.