Actually, aliens can't document a vessel in the US, even if they set up a corporation. The corporation must be something like 51% US-citizen-owned to document a vessel. (The % is a little more complicated but effectively that.)
And Delaware MIGHT require a boating safety
certificate for whoever rides the jetski in Delaware, you'd need to check the laws on that.
Then if the jetskis are being kept in other states for any period of time...they may require a transfer of registration, typically after 90 days.
And I suspect jetskis are considered too small even for a "recreational" federal documentation
to be issued for them.
All good reasons to find out the Delaware laws, to contact the Delaware authorities and see what else will be required of the jetski owners and users, including safety equipment
and a possible ban on night operations.
As icing on the cake, the registration may also require a Delaware "residential" address. Some states do, other's don't.
The OP's got a project
ahead of him, probably a full day on the phone
PS-The advice or information that a bureaucrat or administrative agent gives you on the phone
, or even in person, in the US? Is not legally binding, to be certain if it is correct, you need to find the authorized legal
printed version of the laws in question. Or at least, the official agency's printed web site.