Falcon, please don't take this as criticism, but why are you a mediocre cadet? Is it because you don't want to be at USAFA and thus your performance is mediocre? Is it the academics? Academics at any SA are tough.
What do you want to do in the Navy that you can't do in the AF? Obviously the AF doesn't have ships or subs, but they have lots of other similar jobs.
Control what you can control, which is being the BEST cadet at USAFA that you can be. Don't take the "you can't cross commission if you aren't a star cadet" -- become a star cadet.
Apply for semester exchange. If the job you want is only in the Navy, see if there is a summer program you could do if you give up one of your summer blocks.
DS knew of 2 classmates who cross commissioned Navy -- one was good cadet who really wanted SEAL and he worked for it, did summer program geared for it, and became a SEAL. The other wasn't a "star" but chose a job in the Navy, geared his classwork for it, and was able to convince people that was where he would best serve the in the military. I'm sure he got backing from faculty and found someone in the Navy willing to go to bat for him, but he managed to do it.