Sorry, I know it's a bit confusing. Basically, Mind Vision allows Anduin the ability to see through the eyes of who/whatever he has cast the spell on (friend or foe). From there, if there is something/someone else in view of the person whose eyes he is seeing through, he would have the ability to cast the spell again and then see through the eyes of them, and so on and so forth. It's not a mind-control spell or any sort of a communication spell, they carry on doing whatever they are doing and he just sees it all through their eyes for the moment, it's very passive. If the target is sensitive to mental manipulation they might be able to sense this happening but most are otherwise unaware of the presence of the spell.

ALSO possibly this might be better to do with birds than mermaids, considering how MISTY it is, or crossing from mermaids TO birds then maybe to the boats themselves? tl;dr a very complicated way of getting a closer look and get some idea of who these people are/what the state of their boat is, I am presuming the telescopes don't give them this information already.