There's lot's of 'boat guys' hanging around marinas...
The question is... is he truly knowledgable about this stuff or did he give it just a quick, cursory glance.
But, let's go on the assumption that there's no underlying damage (which, again, you'll ideally need a good surveyor or glass guy to look at it). In that case (assuming no real damage), those cracks would not worry me about water intrusion. Anything above the waterline, with tiny cracks like that, will be fine. However, cracks don't shrink - they can get bigger... and they
will get bigger/longer over time. If you want to at least stop them from growing, drill a small hole at the end of every crack - just barely through the gel and into the glass only a tiny bit. Then fill the hole. Nothing is guaranteed, but that's about the best way to try and prevent them from growing.
In the end, you need to make the decision - don't rely just on what I am saying. If you're comfortable with it as is, then go boating and keep an eye on them. If it's going to bother you, get it checked.