Surely title to the boat does not pass until the transaction is complete - ie the owner receives his money. If Opal, as sales agent, has "lost" or "diverted" the transfer it is up to the purchaser to recover his money, not the vendor - he still has his boat. Thus the money only transacts through the agent, the agent never takes title to the vessel, and this will be clear in the sale agreement.
I think you will find that the broker acts as the sellers authourised reprensitive, so once money has passed to the broker, in terms of the contract the sale has been deemed to have been completed as the authorised representitive of the seller has received payment.
I also assume a bill of sale would have been signed at the time the money was paid, and as such the vessel is now the posseion of the Buyer. It is the Seller who now has a dispute with their (appointed freely by them) representitve regarding transfer of the funds.
I seem to recall when purchasing through another well known brokerage that, although my funds had cleared into the brokers clients account on the Friday I was unable to take possession of the vessel until the funds where cleared into the sellers own account, this occurred on the Monday. I was mightily annoyed but allegedly title remains with the seller until he has actually received the funds. The broker is effectively a stakeholder and not an agent of the seller.