In the extended rules online, it says that strongholds are cards but that they can't be referred to for the term "card(s)" for cost and targetting.
So could I use a stronghold as one of the actions required to put the Ring of Water into play?
Ring of Water and Stronghold Battle Actions
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
(C)ould I use a stronghold as one of the actions required to put the Ring of Water into play?
Yes.
On: Ring of Water