Many details show that this sentence cannot be ancient (and in fact there is no trace of it in the usual places). While there are no obvious mistakes, it is clear that it is a literal translation from English with no attempt to produce idiomatic Latin. This does not mean that the translator didn’t know enough Latin: the purpose might have been humoristic.
In fact, it is the translation of
Call on God but row away from the rocks.
As you can see, it is a rather popular quote whose origin might prove hard to trace. Some sites claim it is an American Indian proverb. I seriously doubt it, because, as other users have explained marvelously, the idea is deeply rooted in Christianity. I don’t think it is of Southern European origin either: on our shores, the one to pray for safety at sea is Maria, Stella Maris.
There is one particular point which I’d like to discuss in this context.
In English (and many related languages), the normal way to express motion is to choose a verb which shows the mode of displacement, while the direction is expressed by a complement. In English you walk down the hill. In Latin (and in Romance languages) pedibus a colle descendis, with a directional verb and a complement of manner. So, when translating row away from the rocks, it is not idiomatic to use the very specific verb remigo, which is meant for a situation with a very strong focus on rowing. My own translation of the same sentence would be something like
Precibus Deum invocans remis a rupibus navem averte.
The OP asked in a comment:
He canoed all the time and even invented a canoe paddle... so I wonder if he specifically chose that verb remigo for this translation given his love of canoeing?
Well, it’s difficult to say. Most probably his love of canoeing influenced his choice of a quote, but no, I don’t think his literal translation was also influenced.