In reply to richardhopton:
Depends how much you want to spend!
1. pogies £16 gloves you can push an oar into
http://godfrey.co.uk/club/godfreyrowing/rowing-pogies
2. boat seat pad £18 and a lifesaver
http://godfrey.co.uk/club/godfreyrowing/womens-seatgrip
3. sealskin/waterproof socks - various kinds
4. nice wellies (Hunters obviously the best!)
5. agree on sports massage/pilates/chiropractor vouchers etc - backs usually need a lot of fixing
6. Heart rate monitor and watch?
7. If we're talking in the £100s and she's really keen, then look at a Nielsen Kellerman strokecoach or speedcoach
http://www.oarsport.co.uk/electronics/strokecoach
There is rowing jewelery though in my opinion that is too keen.