You might want to consider: figure out what the really lowest water you'd see would be. Tonight may not be the lowest, so consult the tide tables to find the offsets. In fact, you don't need to go at night, you can go anytime and make the adjustments. What boats are on the other side of your proposed slip?

Blair, as a west coast boater you will find that generally a fin keel is the preferred boat out here, so you may have trouble finding a shoal draft keel. Rather than select your boat for the slip, though, it would be better to select the slip for the boat.

Understanding the tide and current tables is an essential element of boating safety on the west coast, as our tides can range widely. Being stuck in the mud in your slip isn't the most serious issue, it is 1) the rock lurking in your weekend anchorage, 2) the clearance under that bridge, 3) having the proper scope on that anchor to allow for the tide rise, 4) choosing the best time to depart so that you won't be delayed by adverse current, 5) etc. In the front of the tide and current tables is usually a description of how to use them. The calculations can be quite arcane, sometimes, especially at subordinate stations. It does take practice.

I'd want a reliable couple of feet minimum. By reliable I mean at the worst case minus tide, plus an extra allowance for seiche concerns. Three ft or more would satisfy my worries most places.

7.5 feet at minus tide should be just fine.

On the west coast we generally don't sail in 'skinny water.' That's an east coast thing. Heck, they don't even get nervous sailing along for hours with only a foot beneath the keel. That would require a serious shift in paradigm for me! Here in the Pacific Northwest we're used to deeper water. Leaving the Seattle Ship Canal's Green "1" buoy heading out into Puget Sound in just 1 nautical mile you have 500 feet under your keel! The flip side of this is that there are places you can be at 600 feet depth and one minute later you can ram your keel into a solid rock obstruction just invisible below the surface. (This happens way too often.)

To answer your question, yes, the rudder does go way down, almost to keel depth. I have had my keel in the mud in a slip - at a minus tide in La Push, Washington - and I simply waited a couple hours for more depth before I backed out.

A rudder is far more fragile than the keel and there are a number of ways to damage it: For example, backing down too fast and losing your grip on the wheel, the rudder will slam to a limit, and the sudden 'broadside' angle of attack of the rudder to the water can bend the rudder post. This is a risk with most sailboats, not just Catalinas. In your situation, backing out of a slip and shoving the rudder into a steeply rising bottom can do the same. I suggest you ask the marina about the 'shape', the topography, of the bottom...if it is flat mud throughout the marina then you will have no worries.

Another thing to keep in mind is the location of the slip. At a marina where the docks are close to and paralel to the shore line and the slip is perpendicular to the docks I always pull into the slip bow in. There may be 6 or 7 feet at the end of the slip, but only four feet at the front end of the slip. This way my rudder will be in the deeper water.