DISCLAIMER: Your Steps and Tasks may be different
Cullen's ship was sunk by a band of river sprites and he really wants his precious cargo of apple brandy.
Cullen Smyth says, "What am I to do? My boat has sunk in the river and the [cargo] lost. Oh what am I to do?"
Cullen Smyth says, "My cargo of fine apple brandy that I was delivering to the King's table. Did you know that Hakon is a master wood craftsman as well as a hunter?"
From the river that crosses through Cotswold Village, follow the Camelot-side south for quite a ways. It is a little past halfway to Prydwen Keep, so it may take a minute or two. You will see a ship surrounded by sprites along the shoreline. Kill some and one named Sephucoth will spawn. He drops the brandy. Bring it back to Cullen and he will give you a gold nugget to sell to a merchant.
*Note: you don't have to talk to Cullen first, just kill Sephucoth and then deliver the brandy. Brandy is a common drop from Sepucoth. If you are going to kill Sephucoth, you might as well also perform the Heart of Sephucoth quest as well.