I have never launched from Looe, but have spent a few weeks round there and often vist for lunch. River is tidal, but in a sib I think you will be able to travel a fair way up the river on a decent tide. From memeory, slipway is wide and shallow, launching will probably be + or - 3hrs HW. If conditions are really good (no idea how big your sib is) a trip to Polperro is fantastic in the boat, you can land on the sand beach right in the harbour if your boat is small enough.
Looe near High Tide
Its a fantastic area, we base ourselves just round the corner in Fowey, well worth the trip (By car) to take the boat up the river, or even to Lantic bay if the weather is nice. You can launch from Polruan with a sib over a small hard beach or if you want a proper a slipway there is one at Penmarlam, but its very steep. A trip the other way to Polkerris is also stunning in the right conditions. If you dont have much power, watch the tides at the entrance to either river, it runs fairly strongly and you wont make much ground in a displacement sib.
Polkerris pub and cafe from the water.
Lantic Bay Beach, only accessable by boat and stunning in the right conditions.