M.C Skill is a rating of the effectiveness with which the aquanaut can use his inherent M.C ability.
New recruits will always begin with a value between 16 and 24. This is awarded upon completion of first month's M.C.-Lab training.
An X-COM aquanaut's M.C Skill has a combat increase cap at 100, but can continue to grow past that through M.C Lab training.
Is increased through use of M.C on the battlefield, and through M.C Lab training. Both successful and unsuccessful psi attempts during combat result in improvement.
Unlike any other skills, M.C Skill is not truly capped. Although you will cease to get any more combat increases once you reach 100, you will still get approx. 2-3 skill points per month as long as you stay in Psi training. (Not that it matters... you'll never fail a mind control with a Skill of 100, if your Psi Strength is fairly high.)
Also note that M.C Lab training (for a given month) is lost if the soldier is transferred to another base, even if the destination base has M.C Lab space available.