If you ‘make a splash’, you get a lot of public attention.
• We need to make a splash by holding a cocktail party for journalists.
• She made quite a splash when she wore such a small dress to the film premiere.

If a noise is ‘drowned out’, you cannot hear it because of other noises.
• The sounds of the telephone were drowned out by the noise from upstairs.
• His speech was drowned out by the chanting from the demonstrators.

If you ‘test the water’, you try to find out what you or people think about an idea or a situation before you take action.
• Before you decide to sell your house in England and move to Spain, why not go there for a trial three months to test the water?
• This is a big project. We should test the water before making such a large investment.

If a situation is ‘sink or swim’, it either fails or succeeds.
• Either this works or we are all out of a job. It’s sink or swim.
• You’ll get no training here. It’s sink or swim.

If you ‘dive into’ something, you do it without really thinking about what you are doing. Also ‘dive into it head-first’. This highlights the lack of preparation and thought.
• He dove into the project with a lot of enthusiasm but not much thought.
• Let’s take our time. There’s no point in diving into this without thinking.

If you are ‘treading water’, you are staying in the same place without making any progress.
• I’m just treading water, waiting for a job with a better salary.
• People lose motivation if they think they are just treading water in their careers.

Can you think of other idioms to do with swimming to add to this list?