If your flight arrives on-time you should have no problem making the 6PM ferry - you won't have time to catch the earlier one that leaves at 4PM.
Your other option is to take a taxi to Reggae Beach and take one of the water taxis over to Nevis. You can probably share the cost of the land taxi to Reggae Beach with other on your flight . Contact Captain Winscent Perkins at Island Watersports for details: Phone: 869-662-7081 or email: firstname.lastname@example.org or check out them on Facebook.
The Island Watersports water taxi schedule is also posted just a few messages down here under "Water Taxi". It may change between now and the end of January, though, so best to contact Winscent the week before you arrive.
Yes I have done this myself