I have things made for a living, sometimes it's a physical thing, sometimes it's an intellectual thing and sometimes it's in between (software! what a nightmare)
The rules within my company are pretty similar to any other, If I'm going to spend more than £25k on a project then the contractors have to go through our full financial health check, if they don't want to go through the healthcheck then they obviously don't want the work. If it's less than £25k I may still make them go through the healthcheck if my spidy senses tingle about any aspect of their business - it's a risk limitation exercise.
Ask for the last 3 years accounts, a letter from their bank manager stating that they are not in any financial difficulty, confirmation from 2 or 3 creditors (normally suppliers) that they are paying their bills and have no outstanding debt other than their normal agreement, at least two character references and two product references from recent customers. Now this might sound like a pain but it is really very simple and takes only a few days. If they won't provide those minimum references (or at least the significant bulk of them) then walk away.
Even when they do provide those guarantees you need to minimise risk, that means you pay the absolute minimum up front..... ideally nothing until you are satisfied with the product. In the real world with small business' this is almost always impossible, agree a deposit and then pay the rest upon satisfactory receipt of goods or the bulk on delivery and say 20% after an agreed period during which time satisfaction can be assured. There is a standard boatbuilders agreement that I've had several boatbuilders bring up when I've discussed payment terms, I can't say that I'm entirely content with it as there may be significant periods of time (and money) when actual ownership will be difficult to establish for items fitted or work carried out on your property - if you've ever dealt with a receiver you will know what I mean. It's not a bad agreement but by no means reduces risk to the absolute minimum. If you absolutely can't risk the money, and if I'm honest this is the only way I would do it, pay a deposit - 10%-15% seems reasonable, 70% on receipt of the boat when you take ownership (and the builder transfers ownership to you!) and the balance after one month only when any problems have been addressed. Draw it up in a milestone agreement as part of a contract, spend time writing the contract to specify exactly what you want and expect and when ownership will be established (I appreciate that most people will have difficulty at this stage as they have no idea what constitutes a contract - a visit to a lawyer at this stage is probably worthwhile and for a few hundred quid it may save you a fortune, all of my work contracts are reviewed by a barrister who specialises in contract law and it generally takes less than an hour of his time)
You are going to get a few nodding heads from "experts" and boatbuilders alike but at the end of the day you are the customer, it's your money.... do not under any circumstances allow the contractor to dictate the terms of your agreement unless you are happy with those terms and understand the risks associated, both parties sign a contract; it is a legal agreement not the builders terms of payment.
Someone further up the thread also mentioned paying by credit card, what a brilliant idea if you have the credit limit, why risk your own money when you can risk somebody else's. If a stage payment system is agreed in whatever form use the credit card to make the payment and then transfer the cash to the card company before the interest period, under UK law credit cards offer added protection in the event of no, or even unsatisfactory goods.
Job done, a reasonably low risk way of purchasing a boat. Some might say what a pain in the arse but if you were buying a house at similar cost you wouldn't just wave your checkbook and pay for it like it was a TV...... would you?