I don't hold NEBOSH and I've never paid anyone with one to do a risk assesment for me. So I'm not going to answer your question directly, but I work in an industry where people are obsessed with competent risk assessment and spend silly money getting them done, often with no benefit to either safety or the duty holder. I have however seen the output from NEBOSH qualified people and it is hugely variable.
Safety legislation at sea may be slightly different but since you are looking for NEBOSH I assume the framework is the same or identical. On land your risk assessment has to be "suitable and sufficient", unhelpfully its generally not defined what those terms mean. Responsibility for ensuring it is suitable and sufficient lies with you (the duty holder) and you can't contract out of that. Obviously you can argue that you brought in appropriate expertise and competent help to assist - but its your responsibility to ensure that external advisors have suitable experience, and competence to provide appropriate advice (so get and keep copies of qualifications, CVs and references - if the shit hits the fan this file is an important part of the sentence which begins "Your honour, I did everything I reasonably could...").
Now, there are various reasons why people prepare written risk assessments. There are also various reasons why people bring in external help to do that. Whatever your reasons, I would strongly suggest you are a very active part of the process for the following reasons:
(1) you probably have a far better idea of what actually goes on than the risk assessor will from "wandering round".
(2) you should give them the truth - a more juniour member of staff is likely to tell them the way your ops manual says its done, even if in reality it never is.
(3) the issues they identify (and the control measures they suggest) you are going to have to live with for some time - with a written record of their recommendations its a real bugger to ignore them ("Yes your honour, I decided I knew more than the expert help I hired in
"). You should debate any issues with them up front. If they put something in writing you still don't agree with write back to them and say "as we discussed your suggested control measure XX is unlikely to be practical in our operation, we intend to manage this risk by YY".
(4) YOU need to own the risk assessment as a useful/meaningful document not a file in a cupboard. All your staff need to know you believe what is in it - not that it is a document to keep the legal people happy once a year during an audit.
Beware "tick sheets". Checklists can be a useful aid memoir but simple Yes / No questions are unlikely to be a suitable and sufficient risk assessment for all but the most mundane tasks - but some assessors like them because they can fire through lots of assessments in minimal time. Equally beware anything with masses of boilerplate guff, it rarely says anything useful and puts people off reading what they need to.
For your operation the main risks are presumably "at sea" so I'd expect much more detail in the areas where the risk lie (e.g. general safety in your office may be very simple but the boat could run to many pages), beware anyone who seems to put as much emphasis on someone falling overboard as someone getting eye strain in front of the computer.
I'd suggest for your operation you want someone who had experience risk assessing marine operations (preferably with the fare paying public). Their credentials in this area would be far more important to me than their NEBOSH paperwork - but if an insurer or someone else is insisting on the ticket then you may need to tick that box. I'd be suspicious of anyone who going to do it in less than 2 days unless they have done something very similar before (in which case be suspicious of cut and paste - is it really YOUR risk he's talking about or just the generic risk of being on A boat). A proper understanding of what you do and how you manage the risk will take a day on site with you and reviewing your manuals etc. It will then take another day to write it all up, discuss it with you ammend bits and present it all in a clear, detailed but consice format (I'd look for a one page summary on any lengthy document). If its just covering the wet side of the operation it might be quicker, but then he'll spend time covering his ass, by writing detailed "scope" statements. You can probably get people who will do this for < £400 all in. If you need it to tick a box then I suppose the cheapest is OK, but if you want it to be of value anyone that cheap is probably not producing something thats really that useful.