Owners Of Boats

What's the task given? 

First let's rewrite all the clues here.

1. Each friend owned two boats. 

2. No friend had two boats of the same color.

3. Alan didn't have a yellow boat.

4. Brian didn't have a red boat, but did have a green one.

5. One of the friends had a yellow boat and a blue boat and another friend had a green boat and a blue boat. 

6. Charles had a yellow boat. 

7. Darren had a blue boat, but didn't have a green one.

Let's make 2 set of 4 colors of boats. Below is the table with owners in row & color of boats in columns.  

Below are clues which giving clear idea of owner of particular colored boat.

3. Alan didn't have a yellow boat.

4. Brian didn't have a red boat, but did have a green one.

6. Charles had a yellow boat.

7. Darren had a blue boat, but didn't have a green one.

We will fill this table one by one as per clues given.

Now, consider the first part of this clue.

5. One of the friends had a yellow boat and a blue boat.

Now Alan won't have this combination as he doesn't own yellow boat at all. The Brian already had green boat, so he too can't have this combination as in that case he would own 3 boats.

Let's assume Charles had this combination then other would have boats as below.

Now this eliminates the possibility of having green & blue boats combination. Hence the other part of 5th clue (another friend had a green boat and a blue boat) would be false.

Hence the combination of yellow & blue boats must be owned by Darren.

Putting it in table, we get,

Since 2 yellow boats are owned by Charles & Darren & Brain already have green (but not red for sure) boat, color of second boat owned by Brian must blue one.This makes later part of clue 5 ( another friend had a green boat and a blue boat) true.
 

Now Alan is left with only possible combination i.e of red & green boats.

 Consequently, Charles is going to have one of the other red boat.
 

 To conclude,

Alan had a red boat and a green boat.

Brian had a green boat and a blue boat.

Charles had a yellow boat and a red boat.

Darren had a blue boat and a yellow boat.