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Most marine engines will have a raw water pump with a flexible impeller. It is used to pump raw water from outside the boat through the engine's raw water cooling plumbing. These impellers are very reliable but some care is required.
They should never be run dry. It only takes a very short time for them to disintegrate running dry. Typically when they're replaced or the pump housing is opened for inspection, they should be lubricated to give them a chance to suck in the water initially without abrading against the sides of the pump. Once water starts flowing through, this will lubricate them. Some manufacturers claim that oil based grease is bad for the rubber or other material of which the impeller is made and suggest using something like a dishwashing soap or other proprietary lubricant. Others say that oil based grease is fine. Many mechanics use oil based light grease because it doesn't last long after water is introduced to the inside of the pump body.
When inserting an impeller, be sure to use durable water proof grease or other lubricating agent suitable for the purpose and approved by the pump manufacturer on the shaft, so that the impellor will be less likely to become seized to the shaft over time and will be more readily removable when the time comes to do so. Also be sure to replace the gasket or O ring sealing the face plate that covers the impeller. Many manufacturers recommend spreading a little grease on it. Spreading grease or lubricant (as the manufacturer recommends) on the face plate will help to avoid dry start up friction where the impeller moves against the plate.
Often it is very difficult to get the old impellor out. Some mechanics pry it out with screw drivers but these can damage the lip of the pump housing if you're not very careful. The best bet is to buy an impeller puller that fits your pump. You may find that the clearances around your pump don't give you room to use that tool, but generally it's a good idea to remove the pump from the engine anyway for ease of work and better inspection. Inserting the new impeller into the pump is also often difficult and takes some practice. You basically have to compress and squeeze the blades as you twist the impeller into the pump, preferably in the direction of turn by the engine, until the splines on the shaft engage with the impellor hub. Again, this is much easier to do if the pump is off the engine. Some pumps have a key in the shaft which must go into the impeller hub without falling out. These are very difficult to work with. After changing an impeller, run the engine for a while and check to be sure there is no seepage around the face plate or anywhere else.
Always carry at least one spare. Different companies will probably make impellers for your pump. You seldom have to buy one made by the pump maker, although this may be a good idea. Always look for quality impellers.