I went to Roatan this January--the last week in Jan to be exact. I got a great deal, which is why I went--and the resorts are offering the same great deals this next Jan, which is probably why you're going in Jan!
But the 3-4 weeks prior to my trip this year, it was raining there almost non-stop. The airport in Roatan closes if it sprinkles, and for the weeks leading up to our trip, we kept hearing nightmare stories about people not being able to get off the island and having to pay for extra days at their hotel until it quit raining.
Luckily this didn't happen to us, but it rained a LOT the week we were there. Out of 7 days, we had 2 days with ANY sun and it was sporadic at best. The rest of the time it was just cloudy and windy. When it rained, it didn't last all day--it would rain, then stop, then rain, then stop....and a lot of times it would rain at night. My favorite was when it would start raining while we were diving--I got a great picture of the rain hitting the surface from underwater. Very cool! I would definitely take a rain jacket or poncho, at least for the dive boat--it can get pretty chilly with the wind blowing the rain on you when you're wet.
The water was pretty choppy from the winds and some days being on the dive boat was like riding a bucking bronco. We were on the south side at Fantasy Island and were able to dive every day. We only had to cancel one dive because a bad storm was coming in but otherwise did all 3 dives every day. We toured Anthony's Key resort on the north side and the water was so rough over there, none of the boats were out. People on the plane who stayed there said they hadn't been able to dive much that week.
The air temp in Jan was ok--it was mid 70's and humid, which was comfortable during the day but I was cold from diving all day every day and had to wear a jacket at night. The water temps were a pretty consistent 78 degrees that week--I froze on every dive but most of the other divers seemed fine in 3 mils.
Basically I had a great time and would love to go back but the only way I would go back in Jan is if I got a great deal. The locals I talked to, and my TA at Fantasy Island who is from Roatan, all said May is the best month to go. But weather in the tropics is always iffy so just book it and go have a great time!