Would this model be good for combat sports (mma/wrestling/jiu jitsu)?

I’m not sure , I do run my Spartan Races in them . I guess you could try , try are perfect compression leggings. I do like them .

Can I use it for swimming?

It is not recommended to wear the 2XU men's core compression tights while swimming, but they are ideal to wear pre-swim for a faster warm up and post-swim workout to aid and speed recovery. Product care instructions are machine wash in cold water and hang to dry. The 2XU men's core compression tights are not treated for use in water. Please note that the manufacturer warranty does not cover damage caused by incorrect use or failure to follow care instructions. If you still wish to wear these to swim be mindful that salt and chlorinated water increase damage & decomposition to materials and seems. While swim damage is not covered by warranty, you may be able to maximize product life and minimize damage by rinsing thoroughly with fresh cold water immediately after swim use.

Does this help for 2 degree centigrade temperature? Or should I opt for the thermal one? I need one for a marathon.

This is a very "it all depends" question. I typically start running in thermals at about the temp in your question. However, I don't run marathons. I have run in non-thermals from 2XU, CW-X and Skins down to around 30F (say -1-ish C) fairly comfortably after my legs have warmed up. I think I'd run them in even lower temps, provided there was little or no wind, but as I have thermals from all the brands mentioned, I run those out of habit. My suggestion would be to go with CW-X Stabilyx (non-thermal) as those are heavier to start with, plus the support system is great (for my knees, anyway); the CW-X thermals are almost too heavy and stiff unless the temp is down around the low 20sF. Having said that, 2XU compression compares very favorably with the other tier one brands (e.g., CW-X, Skins). If you are looking for compressive comfort both during and after a run, these are great. The edge 2XU has over CW-X is the thermals are surprisingly light ("are these really thermals?") yet, once on the road, they do a great job overall. Respondent demographics: 55+, 15-20 road miles per week, no marathons of any length, some trail running, but mostly road work. (Good luck to you.)