He is talking about the lady going down to the riptide (i.e That she's taking a chance [perhaps in the entertainment industry]) and by this she's gone to the darkside (suggesting that she's desperate and she's gone there through dubious means, though 'taken away' suggests that she instigated it but it eventually became out of her control).
A riptide is a tide that goes against other tides, suggesting that she's had many issues which hurt her and the riptide denotes an opportunity (something that opposes her current problems) - perhaps the reason for her desperation.
By saying he wants to be her left-hand man, he is showing that he will follow her no matter what (even into the darkness); left suggest the opposite of what is good, hence he's willing to give up morality to be with her (he's so infatuated that he doesn't care to actually help her from the dark side)