They say that prior proper preparation prevents preposterous performance. That saying is very applicable to the marine environment. Salt water is very corrosive and in regards to pumps, it is the impellor and seals that takes the brunt of this corrosiveness. With proper pump maintenance, you can avoid costly damage to your systems and keep your vessel in top form.
In general it is good practice to replace the impellors in your pumps once a year, no matter how often or how many hours you use these pumps. The cost for repair from an impellor that has gone bad can be steep, as they disintegrate and can clog systems where they are in place.
There can be many impellor pumps in your boat. There is usually one in your main motors cooling system, in the raw sea water pump. The generator will also have this same system if you have one aboard. Water pumps can be, and some bilge pumps can be. The rubber impellor pumps are the ones that need this care and concern.
It is always a great idea to have spare impellors and spare seals on board for the random incident that can take place at usually the most inappropriate times. When you think about it pumps are some of the more important devices on the boat, it pays dividends to give these simple devices the attention they require for proper performance.