If you are having problems with describing sound leave it to the imagination of your readers - just state what is causing the sound.
If your characters can see or simply know that it's an oar hitting the water he can state for example how loud the sound of the oar hitting the water is or how the water splashes up while he is rowing the boat. Your readers will know that water splashing is not completely quiet and they will likely have heard something similar before in their life.
If you desparately need to describe something you can simply compare it to something you find easier to describe and that fits the mood. For example you could compare the sound of the oar hitting the water to the sound of a stone falling into a pond or if you need a darker mood as if someone's head was pushed under water only to be pulled out again a few seconds later. It doesn't really matter if it's a perfect description - if it fits the mood your readers will be able to imagine something and you can move on with your writing.