Ok, while I'm still troubling with
sending extra data (via PHP) in eForm, now I encountered another problem.
The goal is simple:

1. From the drop-down menu select some boat (attached image how it looks)
2. Click to define the desired date
3. Select the duration (1 week, 2 weeks, etc...)
4. Click the button and send the "user-selected" data to eForm
The solution can be made via jQuery, PHP...whateva... but PLEEEEASE be specific
I'm using eForm for years now, but I never used it with extra data... Always they were some simple contact forms...
and one more important thing....
THAAAAAAAAAAANK YOU!!!