We went to Porto Mar for our last night in Montreal and it was excellent. We had a chance to try a number of different things (we got the mixed seafood grill for two) and we liked everything. Honestly, the sardines require skill to separate the meat from the bones and we weren't very good at that. But really, you can't beat the food here and it's not the restaurant's fault that sardines are built the way they are.
The odd beauty of Porto Mar (at least if you're a customer) is that it never seems to be crowded. This was true when we were there and it's also been true for many of the other reviewers, so take advantage of this. Go when the old city is full of tourists. Go when you're tired of the noise and the crowds. Go early in your trip so you can go back again. Can you tell I liked it?
There's apparently an architectural dig happening nearby. There are signs that mention an old fire, there were ancient walls being unearthed, and it was interesting to walk around the area. But back to the restaurant. Go there and recommend it to other people, you'll look like a genius.