Running ink and blurry text.
Too literal? Ok.
I'd think it would be a series of situational Snags and/or penalties instead of 'mental hit points'.
For example, someone on the rowboat got messed up and nailed by an insanity attack, failed save, and now suffers. (Maybe it's only temp) Until it's removed (time, healing, drugs, etc) he refuses to go on boats, needs a will save to do so when necessary, and suffers a -2 to all actions when on a boat. Maybe it gets worse after another bad incident on a sailboat. Now he's at -4 on boats and to his will save to temporarily overcome his fear, and it's so bad, he even gets a -1 if near bodies of water. (Something large and deep enough to swim in, not the bathtub.)
Just an idea. I remember trying to keep track of sanity points in many games (including Call of Cthulhu), and it never seemed to go as easily as HP did. Besides, the only part that really mattered to the players was when you actually went nuts, not the point based decent into madness.
Hope my opinion helps rather than confuses. If it causes insanity, I'm not responsible, it must have driven me insane when I thought of it.