My boat is stored is stored in an unheated garage. I put a small,t-stat controlled electric heater on the floor under the console. I turn it to the lowest setting. The heat rises and keeps the water pressure and speedo from freezing. The only other sure way I know of is to remove and store inside. I don't trust bleeder valves.
that often won't work. The tubing needs to have a straight run to the drain. But under the dash, it is generally lashed with or looped around all sorts of stuff leaving high and low spots. If the tube runs above the speedo connector barb, water won't run out and it will freeze. Best solution is to disconnect at the speedo (usually held by a good zip-tie) BEFORE freezing temps hit. It should drain out of the speedo. I would use modest air pressure on the tube to blow all the water out, particularly out of the lower unit which is the only other point you really care about ice forming.
I installed a 'T' drop line with valve in 2003, similar to what I saw on the internet; between console and engine shifter; made sure there were no loops in water line. I live in Missouri where it gets below zero sometimes. I haven't had any frozen water pressure gauge problems in 13 years. Just open the valve, and water drains out. Just have to remember to close valve in spring.
1997 Mercury 175 EFI / 1985 184 Champ / 5.5" manual CMC. The cable below the valve goes to the hotfoot pedal. Boat is stored in unheated garage. I bought boat in 1989; in 2003 I had local shop replace original steering (cable broke) with NFB steering, add CMC, add water pressure gauge; I put in the 'T/valve' deal. Luckily I already had this picture on Photobucket and didn't have to go out into freezing rain to take this picture.