It's true that "once you get him in, you gotta get him in" only to a certain extent ...
you can more accurately say, "Once he outright refuses to go in, you gotta get him in."
The way we tricked my horse into going through water was to lead him up to it as if we weren't going to make him cross. We just walked him around and up to the water but didn't force him to step in -- just let him 'hang out' near the scary thing until it stopped being scary, then made him look at the scary thing until it stopped being scary, and then encouraged him gently to walk through it until ... well, you get the idea. : )
Once he steps into it, let him walk around in it and get used to the idea. Eventually you can ask more of him, like "So we're in the water now, can we walk all the way across without having to paw or stop to sniff?" And from there you can get businesslike and demand performance.
Of course, if he ever outright refuses to go in, start over until he does it right .... then let up the pressure and give him a little break. Baby steps ... baby steps.
In my experience, trying to force a horse to do anything is just going to make them more afraid and less likely to do it. Making it fun and non-threatening will encourage them to have heart and be willing to follow you anywhere.