For stability buy one of those cheap "noodles" or use the horseshoe life device.
Easy warm up and cool down, either tread water (on warm up you may like to go a bit faster) or put your hands on your hips and make wide frog like hip rotations out then in. Feel free to make frog noises if it makes you happy!
For both circle your arms with full range of movement, again out then in.
Aim for 5 mins.
Here are just a few exercises you can do, I will add more as soon as the water warms up enough for ...
