But where is the root cause of this problem? 

The previous owners had a $7k system put in including a sump pump and perimeter drain to prevent such an occurrence from happening.

The Wall Street Journal just released announces that the Biden administration announced a three-month extension to a national eviction moratorium, a move designed to help millions of tenants who have fallen behind on their rent even as courts have disagreed on whether the relief is legal.The Centers for Disease Control and Prevention extended the eviction moratorium through June 30.

Is it worth Putting up a Gate around a pond in order to prevent potential liabilities and future lawsuits..... any other suggestions on how to keep the pond and mitigate the risk. 

I would go ahead and start the eviction process even if the moratorium prevents it from going all the way through.

My wife and I along with our two children are pretty rooted in our current community and love it there, but are very interested in REI. 

Does the moratorium prevent me from notifying the tenants I won't be extending their leases? 

States vary on what you can get away with here, but so long as you don't excuse yourself from intentional acts or gross negligence then in most states its ok.Whether or not you can get someone to sign a contract with these clauses in it is another story.Also, this won't prevent you from being sued.

Many had something really strange with them such as 1) built on a stilts on a slope above hwy 163 with literally a stamp sized flat area of the parcel or a unit that was attached to another unit and the next door unit had a door that accessed a lounge area on the roof (the only door that accessed the area) 3) Had no access to the house and what once was used as access the owners had built a pile of trash to prevent access to the property (I suspect there had not been an easement there). 

That prevents some people from taking the step.