Tell ya what---I've used the 704 as my go-to spinning reel since I was a teenager. First reel of my own I ever bought (still have that greenie in the basement). That drag is damn near bulletproof, and especially so when you're running HT-100 washers. I've cleaned mine after seasons where hundreds of fish came to the beach or the rocks. Sometimes, the metal washers would be scored, with concentric alternating rings of brass and chrome, like the growth rings on an oak tree. Sometimes, they'd have rust spots, or spots where the chrome flaked off and raw brass was showing, that were the size of a ladybug. Sometimes, the old leather washers would be pasted to metal and come off in pieces. After the HT-100s came out, sometimes they'd be salt encrusted. But, right up till the end of the season, the drag never missed a beat---never stuck and never seized. As long as the metal washers are in the right order, and as long as there's bits of fiber between them, it works. But, put two of those metal washers in the wrong order and she'll lock down tighter than Fort Knox within 5 revolutions of the spool.
And, why I think your shop guy isn't correct is, if your line was that loose---
and believe me, the ONLY way it could ever be loose enough for 15# or 20# mono to dig and bind was if it was wound on under no tension at all---you'd get the mother of all birdsnests within your first three casts with a snake
If you're casting an eel and retrieving it slow, there is plenty of tension on the line when you're reeling in. Enough to prevent what your shop guy's telling you the problem is. Even if your line is getting pushed by incoming waves.