I used a 3gpm water transfer pump that ran fine for about 10 minutes but then lost it's prime & started smoking. What have you used successfully?

I used an inexpensive 1100 GPH submersible Bilge Pump (Brand was Maxzone) hooked up to a 12 volt battery. Ran it for 4 hrs. +, worked great, and pump still works like new!

can you dilute this and pour directly into thermostat housing of a Yamaha 150 OB? I can't take my boat out of the water

Yes, manufacturer recommends 1:1 dilution with clean water. Static cleaning may require additional time for cleaning, typically 1-2 hours, may require 3 hours at most.

can you dilute this to 1-1 and pour directly into thermostat housing? I can't take my boat out of the water

Yes. You didn’t say what type of motors you have? I made a setup to do a close loop system. Remove your thermostats and reinstall housing. Connect a hose or tubing to thermostat housing and run it to a bucket with a small 1/4 hp water sump pump in a bucket or a 30 gallon trash can is what I used with cleaner solution and then connect a hose to the out flow and run that to the bucket so you can get a full circulation through your cooling system. This set up is I used when I can’t run the motors. Don’t by the pump kit they sell. It’s junk. The pump is way to inefficient. The cleaner works really well . Only takes about 30 minutes. You can run longer time but You will be amazed how much crap comes out in a short time. You will need to completely flush the cooling system with clean water after you are done. Install new thermostats. Thermostats are cheap. Don’t try and just clean the old ones. If you have outboards and can take boat out of the water that’s the easiest way. I have a tank that I can lower my motors in and pour the cleaner solution in the tanks but I still remove the thermostats and reinstall the thermostat housings and run the motors for about 5-10 minutes. I do this every 50 hours or so. I run my boat in saltwater. You need to watch your temperature gauges constantly so you don’t over heat the motors. The water in the tanks will get hot.