Yep,... Need more details. I don't know that I'd automatically say it isn't safe. Depending on how you intend to use the boat and what you expect out of it there may be things you can do.
If you are expecting an almost 'invisible' repair, and want to use the boat in extended open water then your options may be more limited. If it is pretty much a knock around boat for local exploring and gunk holing then there are probably lots of things you can do. There are lots of products (latex caulking, construction adhesives, etc) that will keep the water out.
While growing up we had an old beater 12' aluminum john boat ( actually bought it new, but after 20 yrs of hard salt water use it was pretty tired.). In the spring we would go over the bottom wire brushing the loose rivets, cracks and anything else that looked like it might lead, then we would apply a dab of whatever we had laying around the house. It would be good to go for the season. By the end of the season things would have loosened up again to the point that after a couple of hours in the water there would also be a good couple of inches in it. It was never anything we would have called 'unsafe'.
Had lots of fun with it, at times we would take it out and intentionally roll it over, climb on the bottom, swim under it to the air pockets in it and generally use it as a water toy. When done we would role it back over, slosh a good bit of the water out, climb in and row back to shore. More than a couple of times a passing boat asked if we needed help--- just having fun.
