Translation:The engineer works with the boat.
"On the boat" would be "ar an mbád" wouldn't it? An engineer working on a boat implies he is fixing, repairing or maintaining something. Working with the boat would imply he is working on its behavior in one way or another, perhaps to make it go faster or be more stable in a crosswind. For instance one often works 'with' sails but seldom works 'on' sails.
My hangup with the sentence is - as with a hundred before it - spelling. I don't see why the "t-" in "t-innealtóir" is strictly necessary.
"To work with" seems an odd construction to me, in this context. I can work with people, but I work on machines, whether to maintain them or to improve their behaviour or performance. Perhaps "working with" a machine to improve performance or to elicit an improved response is an American usage. I have not heard it in the UK. All the mechanics I know (straw poll) "work on" cars.
As for an t-innealtóir - it's a masculine singular noun beginning with a vowel, following the article, and it is in the common case as the subject of a sentence; so it gets the t- treatment, like an t-am, an t-im, an t-arán. Feminine nouns are unaffected in the common case: an eochair, an aintín, an oíche.