The swamp lands of Britannia are very dangerous. Everyone walking through them risks being poisoned, which means a slow and painful death for the victim. Since red Potions or a Cure spell aren't always available, these boots were developed for travelers and adventurers. With swamp boots, there is no danger of being poisoned when standing in the waters of a swamp, as long as no other parts of the body touch it (Ultima IX concepts excepted). Swamp boots protect from the poisons in the water and the poisonous slugs therein. Therefore, every adventurer should have one pair of swamp boots ready, as they are a guarantee that their life will last a lot longer in these badlands.
In Ultima VI, swamp boots could be bought for the small price of 10 gold at typical armouries, such as that of Utomo in the village of Yew. Similarly, in VII they could be bought cheaply at provisioners (Britain & Trinsic) or found. In Ultima IX, however, they were a special item and only one pair could be found. In Ultima IX the Avatar could also be completely immersed in swamp water, wearing his swamp boots, and not be effected by the poison. This is a property they did not have in previous games.