Genesis Ultimatum is a one-time use since it exiles itself upon resolution
For example, if an opponent is instructed to look at the top card of your library, you should hand them that card, and so on
Put them together, and shenanigans will almost certainly be afoot
If the target creature is an illegal target as Floodpits Drowner's last ability tries to resolve, it won't resolve and none of its effects will happen
Many commanders in MTG can fully station these lands on the turn they come in, and there are lots of ways to double creature power these days, so dont write these off as clunky
One way is to think about it like a control deck: assuming you have infinite time to get the job done once the game is locked up, go for the most irresistible and foolproof plan possible