No you cannot swim in a geyser.
Swimming in a geyser is unsafe because the water in geysers is much too hot for the human body and can cause heat stroke as well as burn your skin.
You could actually die or get severely injured if you tried to swim in a geysers.
Geysers are amazing and pretty to look at but you need to keep your distance from the geyser because they can be dangerous.
If you go to Yellowstone park it's illegal to swim in any of Yellowstone's thermal features including the geyser.
So never swim in a geyser or touch the water in the geyser because you can get severely burned and injured.