The actual lyrics as sung by Josh according to my transcription five minutes ago -- sorry, I know it's taken four years -- are as follows (and if anybody wants the MP3, email me at kyle@scunnered.com):

When the weather is fine then you know it's a sign
For messing about on the river.
If you take my advice there's nothing so nice
As messing about on the river.
There are long boats and short boats and all kinds of craft,
And cruisers and keel boats and some with no draught.
So take off your coat and hop in a boat
Go messing about on the river.
There are boats made from kits that reach you in bits
For messing about on the river.
Or you might want to skull in a glass-fibred hull.
Just messing about on the river.
There are tillers and rudders and anchors and cleats,
And ropes that are sometimes referred to as sheets.
With the wind in your face there's no finer place,
Than messing about on the river.
There are skippers and mates and rowing club eights
Just messing about on the river.
There are pontoons and trots(?) and all sorts of knots
For messing about on the river.
With inboards and outboards and dinghies you sail.
The first thing you learn is the right way to bail.
In a one-seat canoe you're the skipper and crew,
Just messing about on the river.
There are bridges and locks and moorings and docks
When messing about on the river.
There's a whirlpool and weir that you mustn't go near
When messing about on the river.
There are backwater places all hidden from view,
And quaint little islands just awaiting for you.
So I'll leave you right now to cast off your bow,
Go messing about on the river.