Some have been so lucky as to get it back in place without removing the powerhead, but I don't know how. And it will just pull out again next time you remove the lower unit. To do it right, you need to pull the powerhead. The water tube is held in place by a push nut at the top.
Not a job for the timid, that's for sure.
EDIT: While you have it apart, take advantage of the opportunity and replace the rubber boot on the shift rod. It will need it sooner or later, so do it now. Otherwise you will be doing it again later. And take a real good look at the upper front rubber mount-another common failure item.