I keep a "Mastercraft" from Canadian Tire (Canada) unit on board at all times. I connect it to the starting battery so that it is always charged ( I connect after the boat is started for the first time of the day and disconnect at the end of the day)
I have used mine to power up my buddies windlass on his 32 ft Carver whenever the solenoid failed.
Coincidentally. my other buddy (1988 FW Vista 245) has three of these units one attaches to each of his batteries so he has extra house capacity and like me extra starting capacity. The one on the starting battery has an inverter built in and he uses it to power a dvd/tv combo and his laptop.
He bought most of them at yard sales real cheap because people had either lost the cords or were frustrated because they were never charged whenevr they needed them.
I would never rely on one of these to jump start a cold engine in the middle of the winter. They are more for warm weather starts IMO. I use mine alot for the lawn tractor (damn kids leave the lights on!)