I have a very similar set up to you, Excel 365 and Honda 10hp 4 stroke, and have also done the Medway on many occassions.
If you are looking for a place to launch on the sea, a trip down to Sandwich is worth it. You can either launch at the small slipway in the town and take a gentle cruise down the river and out to sea, and then go either right, down to Sandwich Bay and beyond to Deal etc. or go left up towards Broadstairs and beyond, may be landing at Botany Bay. Or, you can launch at Sandwich Bay, just park up in the dunes and carry down to the water, if you do this I suggest you go as far down the end, left as you arrive at the seafront, as possible, as that will be the closest to the sea.
You are very likely to sea some seals in the estuary area and as you start to go up the river, very nice.