What does it mean to dream of floating on a mattress in the water and using an umbrella for a oar?

It means your dreams are a lot weirder than mine.

Dreams may be an insight into the subconscious mind. What they are not is accurate predictors of the future. You should talk your dream over with your psychologist; there's no magical "this means this" dictionary (well, there ARE, but they're bunk).

The odd images seen in dreams are metaphors, which is why they can be pretty weird. In this dream, the focus is the water, probably representing a difficult situation. The mattress and umbrella symbolize the dreamer's means of "staying above water." (That figurative expression is itself the sort of metaphor that dreams picture as literal images.)

.... The mattress might represent sleeping - perhaps the dreamer hopes to wake up and find the problem has resolved itself. Or it could suggest sexual methods - perhaps the dreamer hopes to use sexual wiles or flirtation as a way of gaining advantage in the situation.

.... The umbrella is an appropriate means of protecting oneself from water (rain) but it is being used in an inappropriate manner in this situation. This suggests that the dreamer is using the wrong approach in dealing with the real life situation.