The reason we use Excalibur 800 for saltwater is because the machine is a gold magnet! The Excals just love gold range targets and just seem to work very good on gold in the salt! I believe BBS and DD coils help with that. All in my group have switched to the 800. Some used to use CZ 20s but they found less and less gold as those with Excals... so they switched. In the salt Excaliburs seem to have the right balance/mix for efficiently finding gold jewelry.
I've never hunted freshwater with Excaliburs, but I would think they would work just as good there too. When I tried a CZ in the surf the problems I encountered were constantly trying to GB the 8 inch coil, and the unit was more difficult to swing in the water as the square shape gave more resistance. Plus the CZ wanted to float a bit and gave off constant false signals in DISC with SENS above 5. With an Excal I just set it and forget it- so to me the Excal is easier to use in the surf.
With that said, one should purchase a balance rod for the Excalibur, and I prefer the tones of the older blue-tube machines- though the Excal II is a bit more sensitive. Sometimes a bad thing. To me there is no doubt that the Fisher CZ 21 is a better overall design! The 8 inch 21 collapses without dis-assembly to fit in a suitcase for travel. I like the quick hip mount option of the 21 and the Hot Auto Tune mode. That's how I would hunt in the saltwater with a 21- volume/sens at max in Auto Tune, GB at 1, then target ID in DISC at 0. Plus, I've often played mind games... If I could only have 1 detector for the rest of my life... what would it be? For me the answer is simple- I would pick the CZ 21 8 inch and use it for everything- land or sea! I feel the 21 would make for a much better all-round hunter than the Excalibur.
If it has not already been mentioned, you should try your friends Excalibur first if you can. At least 2 or 3 times. You'll either love it or hate it. But if you find some gold, I bet I know which detector you will own! As far as reliability goes, that has never really concerned me. With the price of gold these days I've lost track with how many times I've paid for my Excals! I have 2 currently and even if they only lasted 1 year, would buy another! They just find the gold for me in the sea! But to tell the truth, I do not like using my 800 on the beach and would prefer the 21 for that. If you will only hunt freshwater, then a 21 would be a good choice. Dang, I may need to get a 21 myself now!