Royal Caribbean Cruise Line will let anyone buy 'Gift Certificates' to use as gifts. You can use them towards your cruise or use them as onboard credit. You must redeem them prior to your cruise as they can't be used once onboard the ship. Go to their website www.rccl.com and you can read all about it. Hope this helps!
The people "gifting" would need to know the honeymooner's booking number.
I've never heard of an "outside" company doing this, but maybe their TA can help?
"A truly happy person is one who can enjoy the scenery on a detour."