RE: Carve 121 set up
Question 1) You can move the mast base all the way back until the "T"-nut is at the back of the slot, but not hanging over into the wider access slot.
This will put the rear edge of the twist plate all the way back until it's near or slightly over the serial number (on some boards, as the serial numbers have moved around a bit over the years).
What year Carve 121 do you have? I did a little research, and since your first post was in 2003 I'll take a chance that you have a 2003 Carve 121.
If you only move the mast foot back until the edge of the twist plate is at the back of the wider access slot, youi can still go back around one more inch.
You can move the mast foot back until you feel a little tab (on some boards) at the back of the mast slot. You do not want the T-nut hanging over into the unsupporting access slot, but as long as there's full contact between the underside of the "T"'s on the nut and the mast slot, you are OK.
Ummmm... I'd have to see you in action, fully lit up on an "appropriate for the conditions" rig/board/fin to tell if the reason your board is not "unwetting" fully is due to the tuning of the board/ rig/fin, or due to how you are trying to sail it (stance, harness line position and length, and perhaps footstrap position.
How much do you weigh...?
What size rig (s) are you using...?
What is the approximate windspeed...?
When you are really powered up, you normally would feel the board trying to "tailwalk" a little with the mast foot all the way back and (again "normally") you would want to move the mast foot forward a bit to "settle" your board.
What fin are you using...?
Where do you have the footstraps positioned...?
Are you tall, with long legs, or shorter...?
Sorry I don't have a more specific answer, but your situation is so "dynamic" there could be any number of things "unbalanced" and then again maybe nothing is out of tune and you need to place your weight differently, change you stance a little or something similar.
Water coming out "under the front footstraps" is very close to optimum, so you can't be having too big a problem with your issues here.
Hope this helps,