Sounds like your talking about a cir clip to me. I didn't think they used them on water attachments anymore, what brand is this and did it come in a set or did you purchase it separately ? They are made from spring steel so opening up the clip to make it larger will be difficult without the use of two sets of needle nose pliers.
Using needle nose pliers (a small set) while pushing the cir clip with your thumb (try a cheap steel thimble available at any sewing shop/material shop) using your other hand and the needle nose plies try to open one side of the clip while pushing it, this should be enough to get it over the point of no return.
NOTE: don't get any part of your skin caught under there, be a blister in the making if you do
