There are several ways to train endurance by working in the proper heart rate zones, but when it comes to swimming specific dryland exercises it might not be as straight forward.
If you want to work on your power and/or force in your swimming here is a good video I came across, from The Race Club
Lots of good exercises there that will increase your power output in the water once you have access to training in the pool again. If you have access to a medicine ball and some rubber bands or stretch cords you can do most of the exercises in the video. The boxing exercises can be done with someone, or on your own by “shadow boxing” – it will not generate the same amount of force of course but work your core and the rotation.
A nice tool to use while “swimming on land” is the StrechCordz. You can use it for working on details in the swim technique but more importantly you can also use it to build a better force development in your stroke. You can also work with them in a similar fashion to the Vasa Trainer if you have a bench to lie down on while working with the StrechCordz. Shorter intervals with an increasing stroke rate and force development can give you nice gains in your swimming performance in the water. A short study with force measurements and StrechCordz is coming up in the weeks to come. Stay tuned for more specific dryland exercises for improving swim technique and force development!