Kieran,
Try this mob - http://www.yachtandboat.com.au/shop/...ld-baby-infant
If you intend to take your bub out, might be wise to test run her with the jacket in the bath first, then a pool. Not that it should be a concern during winter but keep an eye on over heating & dehydration. Might pay to introduce your bub to the engine noise before you take her out on the water.
One last thing, & I say this with all sincerity - she's got the rest of her life to enjoy the pleasures of boating. Maybe waiting until she can swim might be an option. I'm sure you'll be all over safety and caring for your bub, but it only takes one inattentive moment by another boat, jet ski, unseen submerged object etc to create a regretful incident. All the best.
cheers, HR