FYI - i am also in the midlands i have a lake membership and would use the sea if i could...living in the midlands is tough as the trek to the coast is around 2.5 hours each way so a full day out! i haven't been to the coast this year despite my best intentions but i have been to kingsbury with the kids several times, they love it. i have a Seadoo gtx ltd 215 i bought it 18 months old with 30 hours on the clock. Probably the same money as the new GTI but its loaded and has all the power i need, i was keen to have the IBR system for use on the lake with the kids.
Don't dismiss lake riding its ideal for towing, wake boarding and is super safe for the kids (my children are 5 and 10) i wouldn't want to take the kids out on the sea just yet especially alone. Its also quick and easy for me as i live about 15 mins from kingsbury so there's a convenience aspect to it too, i can pop out ans ski for a morning and be back home by 1 (keeps the wife happy)
Most modern skis are pretty bomb proof so i wouldn't worry about what you buy form a reliability or manufacturers aspect, in my opinion it only really matters what your going to do with it.Your gti is plenty quick enough for your first ski and great for towing and taking the kids out as they are nice and stable so dont worry too much about if you have made the right decision.
like the previous posts say if your still loving it in 12 months and want an upgrade trade it in and go bigger its not a massive uplift.