Several people have mentioned general liability and umbrella coverage in this thread, so I wanted to point out that general liability coverage is generally for bodily injury and property damage - So I think it unlikely that a garden variety $300k or $1M general liability policy would respond to this. 

An injury while riding would be covered by your regular medical insurance. 

Any good attorney in a personal injury will tear apart a waiver/release.

If I had applied, not had sufficient income, and told you I was about to leave my job due to an injury, but had not yet applied for (or had applied but had not yet been awarded) injury compensation yet, I would hope you would tell me no in a heartbeat.

If they want to pay for it and use a contractor that YOU approve, AND:1) they pay for it and reliquish it to you when they leave at no cost to you with no reimbursement to them whatsoever (and SIGN an agreement stating as such... absolutely must sign such an agreement)2) AND they sign a waiver making you not liable for any injuries resulting from the use, install, etc. of the storm shelter....

Recently when in a bind because my regular contractors were busy on other jobs and the backup guy I had scheduled bailed on me at the last minute because of an injury I actually used Craigslist to find new painters and carpenters.Lots of folks responded so I had to be careful who to hire of course. but, bottom line, the carpenter I found is great.

One bad move and you could get sued for a fire, tenant injury,... anything.

They are all misdemeanors because they were spaced apart so I have no felony convictions, and thankfully no incidents that resulted in any injury to anyone. 

Tenants often feel at home in your rental and due to this, seem to do things that are more likely to result in injury or liability issues.

I wanted to take a moment to say my heart goes out to everyone who is suffering injury, loss of life and property.