I don't know where you are. Here I go to the lumber yard, hardware store, home repair store, etc and ask about wire in a well or wet location. (underground splice)
I have shrink tubing that has some kind of jell inside. When the tubing shrinks the jell fills the ends.
I would not use the wire nuts that come with sprinkler systems. I have seen large wet location wire nuts but I don't like them for pumps.
Where you get pumps will have part to make wet wire splices.