Sometimes, especially if you wander too close to the bank, you may get weeds or vegetation round the propeller which slows the boat.
It is sometimes possible to clear a fouled propeller by easing the throttle gear in to reverse for a few seconds, then forward for a few seconds. Repeat this a few times to see if this will clear the propeller.
If this doesn't work you should moor your boat at the bank, switch off the engine. If you can see the weed you can try and clear it with the boat hook. Never go in the water to do this and always make sure the engine is off and the keys are out.
If something larger than weed e.g. mooring ropes has fouled the propeller. Stop immediately and either call the boat yard for help or a diver. Some boat yards offer insurance for a diver callout which can cost upwards of a £100.