If you're staying at this place as a customer, I really don't think you need to care too much about those details.
There are different kinds of special envelopes for money when giving cash to someone as gift or token of appreciation in situations such as wedding, family get-together (eg, otoshidama or money gift to your nieces), etc.
In your case, I think you can hand the money in any envelope... usually money is transferred in envelope that opens along the shorter side (unlike US envelopes which open along the longer side). Unless this boathouse is a really fancy place, you could probably even just hand over the cash as is.