They are for 2 different things. Galvanic isolators stop stray DC current leakage thru the green wire. They stop galvanic current which is anything below 1.2 volts. ELCI is to protect against an AC ground fault leaking into the water if the current load doesn't trip a standard breaker.
I don't have one currently, but probably should consider it. They are a good idea. Shouldn't be too hard to install.
Not where I was headed Pat. What I am about to say is controversial and gets tons of blowback from the usual internet experts but here it is anyway:
With an ELCI installed and tested according to the manufacturer's recommendations, the archaic, ABYC required, stray current attracting AC-DC ground bond is worse than U-S-E-L-E-S-S. Without the AC-DC ground bond there is no possibility of stray current from neighboring boats (see my earlier post about testing with the shore power cord disconnected) and no need for another piece of equipment (galvanic isolator) to protect against the damage the required bond created in the first place.