intermittent tripping checked with meter couldnt find any ground fault

Hey gang, This isn’t strictly a DIY question - I have a professional coming in today to do the work - but before I went forward with it, I wanted to ask what you think. My girlfriend and I moved into a place a couple of years ago with a 26 year old electric water heater. In our first month of living here, we blew a fuse on its 30amp old-timey screw-in fuse circuit. Our first thought was that the heater went kaput so we had someone in to take a look at it and they said that they could replace it with a newer unit but it would probably blow the fuse again so they advised us to get a breaker installed first. Well, cut to yesterday and I wake up with a puddle of water next to our heater in the laundry room. It’s *really* dead this time but we still haven’t had anyone in to switch is to a breaker. Since our heater is a rental, we called the company and they sent out a technician to look at it. He says it’s dead and he’ll put in a new one but, considering the electrical issues we had last time, I decided to look at the fuse box before he came and saw that we have a 30amp circuit hooked to a 12 gauge wire for the water heater. I looked it up online and everywhere I’ve read said that this is a no-no. When I brought it up to the technician today, he said it’s totally fine because the new water heaters have a lower max amperage than our current one so both the fuse box and wire are fine. My question is, are we in danger if he hooks us up to that circuit? If the fuse goes again and we lose hot water, I’m happy to finally call in an electrician and have the breaker put in, but what’s the situation with the wire? Thanks in advance for any help and advice you can give me - it’s very much-appreciated!