Assuming that you're not running any mods, the best foods you will reasonably have available before getting near a swamp are Carrot Soup, Queen's Jam, and either Cooked Meat or Fish.

Before you venture into the swamp, make sure to equip yourself in maxed-out troll armor or bronze armor, bronze mace, and bronze buckler. Bring a hoe to help make easier paths and your swamp key (if scouting for Sunken Crypts). If going on an extended trip, bring along a cart and/or the means to make a portal so that you can pop home for rest, repair, and resupply.

It's always raining in the swamp, which means you will recover hp and stamina slower due to being wet. Don't tire yourself out too much to defend yourself; bring potions and tasty mead for panic moments.

Be on the lookout for Turnip Seeds, which will allow you to start farming Turnips for Turnip Stew, a strong and inexpensive food. Pick up all the thistle you see (needed for several valuable recipes), Blood Bags from leeches (also used in valuable recipes), and collect Entrails from draugr to make sausage.

Unless you're scouring the seas for serpent, you should be decked out in iron gear and eating Sausage, Turnip Stew, and Carrot Soup in preparation for your fight with Bonemass. (Carrot Soup can be subbed out for some form of cooked meat if you want higher hp vs stam.)

All of this is true for about the next 48 hours when Hearth & Home goes live. After that, we shall see.