"Swimming here is forbidden."
Translation:naDev Qallu' net tuch.
"Ongoing swimming forbids it here." That doesn't seem to work.
Is naDev the thing being forbidden or the location of the forbidding? If naDev is not the object, then what is it that is forbidden? Is it only ongoing swimming that is not allowed or do you really want to say that even a moment of swimming is forbidden? How can the ongoingness of the swimming be the thing that is forbidding something? Note that tuch is not an adjectival verb describing something forbidden, but rather an active verb saying that someone is not allowing something.
Did you mean -'egh instead of -ghach? The suffix -'egh means that the subject(-s) is doing something to themself (-ves). The suffix -ghach means "the situation where the intervening suffix applies to an action or quality" (it's very difficult to grasp and not for beginners).
I think you meant the -lu' to go on tuch (the actual verb of the sentence). If you did intend it to go on Qal it would have to go before the -ghach and this sentence would mean something like, "He forbids the continuousness of one swimming here."
I think trying to use -ghach here is a mistake. "Swimming" is an action, not a thing. Figure out how to reword it to use "swim" as a verb!
Officially -ghach is a "nominalizer", but the nuance of it is a little more complex than that implies. It does not simply take a verb and turn it into a noun. Verbs are actions and Klingons are not fond of turning actions into things. Actions are what life is about - things are unimportant. It's probably better to think of -ghach as turning verb suffixes into nouns. So when you attach -ghach to -taH you are talking about the ongoingness of some action.
When I say, "figure out how to reword it", I mean that the particular grammar in this English sentence won't work in Klingon. English is using the gerund "swimming" as a noun, but there is no such noun in Klingon. "Swimming" is a verb in Klingon, so we will need to find a way to express the same idea using a verb. There might be some other ways to also accomplish it, but using a "sentence as object" construction seems the most obvious to me. Your rewording of "forbidden to swim" is a more direct way to translate the needed Klingon sentence.