Some good suggestions here. I only have a couple of things to add from my own experiences.
First, any supplements that will help heal the gut and thus help restore normal digestion and assimilation will treat the root of the problem. I've found l-glutamine, high doses of omega-3 fish oils and George's aloe vera helpful for gut healing and to reduce inflammation.
As far as what you can do to treat the symptom - weight loss - I'd suggest you drop all cardio until your weight has stabilized and substitute a good, heavy weight training program as q suggested. If you must do cardio, 20 minutes of low to moderate intensity, three times a week is all you need in the state you're in. When you run and swim - you don't say for how long or at what intensity - you're working against weight gain and burning energy your body isn't equipped to give, due to your faulty digestion.
I had much success with a weight training program designed specifically for "hard gainers" or ectomorphs, people who have trouble putting on muscle mass. If you're interested in looking into it, you can find more info here: http://www.musclebuildingcoach.com/, but basically you workout only three times a week, working each body part only once, but with compound moves, heavy weight and low reps.
On the diet end of things, I'd suggest you make sure you're getting plenty of protein for gaining healthy lean mass. Boost and Ensure are full of sugars and unhealthy fats. Try a high quality whey protein isolate or egg white protein or even a weight gainer formula as healthier options. Better yet, check into Absorb Plus as a predigested supplement that will help you regain weight. Because it's predigested, even a compromised gut can get the nutrients it desperately needs. Find out more here: http://www.holistichealthshoppe.com/index.php#6 In addition, some other foods that I find helpful are nut butters and avacado, tender chicken and turkey breast and eggs.
Diagnosed with ulcerative colitis spring 1999.
Maintenance dose sulfasalazine.
Probiotics, l-glutamine and fish oil caps. George's aloe vera juice. Oregano oil antibiotic, antiviral, antifungal. Long-term remission with only minor blips.