I'm a freshwater guy so your world frightens me.

Can someone explain the fish finder rig? Looks like a snap swivel that slides up and down on a mono mainline. Why does it slide? Can you clamp it down?

Help a saltwater newb out!

This is a pretty decent rendition of one I found on the internet. I make my own leaders with a crimp tool/set. Instead of a swivel like the coastlock swivel in the picture, I use a sinker slide like this and attach a spider weight to it. The spider weight grabs hold in the sand so your line can't just move around anywhere, although with this rig the bait is allowed to move around a little bit with the waves/current. So basically your bait stays in the same general area due to the spider weight, but is allowed to move a bit since the line has room to move due to the sinker slide. The barrel swivel doubles as a stop for line movement and the beads are knot protection. On some of mine that I make I'll put a wire leader in between the barrel swivel and hook for the toothy critters such as sharks.

Hopefully this has been a somewhat clear explanation and helps you out a little!

Thanks. That is kinda what I figured. What species are these best for?

I forgot to add I'm no pro and haven't been doing this too long, but this rig has worked for me.

It caught me the bull red in my sig pic. Also good for sharks, large rays, and what not. It's the primary rig I use for cut bait like mullet and whiting.

What size and type hook?

Circle hooks. I usually use Mustad, 11/0-14/0 I believe. Gamakatsu makes good ones too but seem to run bigger. Like a 8/0 or 9/0 Gamakatsu is as big as a Mustad in the 11/0-14/0 range. Again, I'm no pro that's been doing this forever so hopefully others will chime for another perspective.

It is pretty much like a bottom fishing rig used for catfishing, but the egg sinker is replaced with either a slider or the swivel/snap assembly so you can clip on different style weights.

Depending on currents, waves, etc. you might use weights ranging in styles from bank sinkers, pyramids, or spider weights, or need to vary the actual weight.

The benefit of using this rig is that the fish does not feel or fight against the weight, instead it will take the bait and pull line tight, fighting against your rod tip (making it a more sensitive rig). If you are using a light drag or bait clicker, the fish will feel a little resistance, but will take line, allowing them to eat the bait enough to take the hook, depending on how you rig it.

Some people make the slider portion part of the whole rig, usually heavier line than the main line to combat wear. Others just run their main line through the slider and tie it on to the swivel.

I have made these to catch everything from croakers up to sharks and everything in between. I vary the size of the circle hook from #4 on up, also the size of the lines, leaders, and swivels depending on the type of fish I plan on running into.