I was intending on avoiding gyroscopes and having the same kind of agility as my supersonic tutorial plane but looking like my first plane with only one boom and engine on each side. thanks for the suggestions.

ok, ive looked at your floating plane, i assume it ends up flipping around. is it buoyant enough to float? if it is not buoyant enough open the overload menu on the fuselage parts (enable overload in mods) then click on the part thing at the top and select fuselage, then increase buoyancy to 10 if that doesn't work 100. so it should be floating at least and very difficult to sink.
,
because the jets are at the back it may cause instability, so add a gyroscope (gizmos) and if that makes it stable then dont increase stability. if this doesnt work, increase stability more in the gyro. if your at max open the xml properties of they gyro, open the tab at the top and go into 'gyroscope' then increase the stability. it will be at 2.5, try upping it to 5 then 10 if its ineffective.
,
i'm not a big fan of using boats in sp so there is definitely better people to answer this question.
the last small scale 'boat' i built was this
which i hope you can use to help you develop your plane.