609 works for me and large diesel bearings but Jdoug would probably have more experience than me with the boat shafts so go with that (no sarcasm intended Jdoug ).
It can be a bit exp and if you can get a small bottle, I would recommend that before getting the big one. Loctite does have a shelf life, I have heard of bottles being rendered useless after about 18months. I get mine for free so it is all good at this end.
As for sweating the shaft with solder, it shouldn't be necessary on the stub end, I usually stick a drill down the stub shaft and rough up the inside a bit and then clean the end of the flex and the inside of the stub with contact cleaner to remove any trace of oil. You then basically fill up the stub shaft hole with the loctite after a good shake of the bottle, it is a good practice to stick a pin into the hole to try to remove any air as well, the loctite is generally pretty thick and you will think the hole is filled when in fact it is all sitting at the top of the hole. Put the flex inside making sure you get lots of ooze of the excess loctite, you want the loctite soaking in to the flex as well hence no solder sweating, put to one side and let it go off. Any excess on the outside won't set and that can be wiped off later.