I see these can be used outside as well, so how do they hold up with standing water? Will the water leak underneath the tiles? Will a sticky mess be left behind?

Any standing water that sits on top of our 3/8" Sport-Lock Rubber Tiles will eventually evaporate. There is potential for water to flow between the interlocks of the tiles if the tiles are not interlocked tightly. Any water under the tiles will not create a sticky mess. You can lift the tiles and allow the water to evaporate as well and then replace the tiles once the area and the tiles are dry.