I would ask your installer to use a dist block that has an extra opening. You'll probably want an amp for the inboat speakers next year. It's probably the same price for the dist block and you covered if you do need it.
I understand about building a system with the parts you have. My system was like that, then over the winter I upgraded. Never intended too, the old system was great, until I teid up next to a boat with a complete Wetsounds system. There is always bigger and better.....