The hydraulic line goes thru the transom behind the top of the actuator, which is above the waterline, and it's about 1/4" in diameter. You drill a hole, feed thru line, hook up your systems, and seal with 5200.
BTW, Todd Bennett has a forum on THT and is very responsive and helpful.
Another BTW, I had a guy show me a cool setup for mounting 'ducers and such on your transom. Instead of mounting each unit you may have, mount one piece of 1/2" color-matched starboard about 4"X12" along the bottom edge of the transom, one side or the other of the engine. Bed it with 5200, using two #10-12 counter-sunk stainless screws. That way you only have two holes in the transom, they are both well sealed, and you can screw as many doo-dads that you wish into that chunk of starboard. If you ever want to remove everythng, there are only two holes to fill with gel coat repair. Some guys even skip the screws and just glue the board to the hull. It's just harder to get it clamped there 'till the sealant sets up.
Now you can have a finder at the back of the boat too, mount a bait tank pump, whatever you want, without drilling a bunch of holes in your transom.