You should really try the bottle of 151 Rum - even if it doesn't work you can still drink it
In all seriousness I have actually used 151 to remove sharpie from plastic and whiteboards - it works scarily well for something that people drink and much better than other cleaning agents I've tried. I haven't tried it on hypalon.
NRS boat cleaner is worthless. To get the most out of the toulene you should soak a rag in it, then drape the rag over the area with the marks - this will soften the hypalon. Then you can use a mild abrasive to remove the sharpie - you will also remove a small amount of the hypalon in the process.
MEK is cheap and painting supply store usually have smaller/cheaper bottles than hardware stores.