There are 8 Lake View GVs at BLT. The main entrance for all the LV GVs is on the 15th floor.
Four of them are between the North Elevator and the North end of the tower, the other 4 are between the South Elevator and the South end of the tower. The rooms on the North end might include a partial MK view, the ones on the South end will face the Contemporary pool and marina.
If you want a lake view that also has a theme park view you can request 8510 or 8512 as they both have great views of the fireworks/castle. 8512 is an "accessible" room if that matters. The upstairs (16th floor) entry/exit door in the theme park GV's allows for direct access to TOTWL without having to go down to the lobby to check in.