Last year I did about 45 Nautical miles on the Clyde in an SR4 / Suz 25. Rhu - Toward - Colintrive - Tighnabruach & back with a cross to Weymss bay to get the "sheltered" side of the firth. The section down the firth was well lumpy, the bit through the Kyles was almost flat. Took all day as the lumpy biots were not condusive to flat out motoring, but did inolve a stop at the cafe in Tinabruach for a large coffee & bacon roll!
As Nos says, if it gets lumpy just slow down a bit and you'll get through it - that's why ribs are so good!