OK. I found it. Looks like I could get RYA Yachtmaster, but I'd be restricted from posting watch or nav duty. Hmmm. Sounds like an OK deal to me.
http://web.pml.ac.uk/solas/forms/msn1765.pdf
"6.3 Eyesight Standards
6.3.1 The approved doctor must also ensure that
the seafarer meets the visual acuity and
colour vision standards and will include
testing during the medical examination. This
includes screening for colour vision, using
Ishihara plates. It is essential that seafarers
applying for certificates of competency as
deck or dual career (merchant/fishing)
officers have full colour vision. A deck
applicant who fails the Ishihara test may
arrange for their colour vision to be re-tested
free of charge, using the Holmes Wright B
Lantern, at one of the MCA Marine Offices
that offer lantern tests (listed at Annex C).
Failure in this test will mean that a medical
certificate may only be issued with a
restriction precluding navigational
watch/lookout duties."