I don't know, but the translation of lōgro āeksio is boat's master.
Good question might be a case of this being a direct translation, and high valyrian not exactly being a fully fleshed out language.
Agreed. I had a look at the High Valyrian Vocabulary here: https://wiki.dothraki.org/High_Valyrian_Vocabulary and it seems there is no word for "Captain". However, I guess the best approximation for Captain would be master, or possibly leader which is "jentys". Hope that's useful.