Aside from the right clothing to protect you from the environment, you'll also need to account for the wildlife. Many creatures in Black Marsh carry disease, and, as the native Argonians are naturally resistant to many such ailments, remedies and treatments are often in short supply, especially in smaller settlements.
Protect yourself from fleshfly bites with a repellent salve, and ensure that you bring plenty of potions to treat blood rot, the droops, and swamp fever. Avoid travel during known breeding seasons for insects and common creatures such as giant snakes, and be very cautious to avoid camping near deep water or anything that appears to be a burrow. As always, it is my advice to travel only with a local guide.