Within the price tier selected, seating is generally first-come, first-served as you arrive, although some planning is involved to ensure parties of different sizes can be accommodated, especially larger parties. Tables fit 2-8. Parties greater than 8 will be seated at immediately adjacent tables.
There is the option when booking to choose to upgrade to a guaranteed private table by the window for $35-60 per person (also includes photos and other extras). Otherwise, especially for smaller parties, seating is likely to be shared with others at a table and usually not along the window, although it really just depends on the makeup of parties on that specific day.
If booking separately than others in your party, please make sure to answer the question at checkout regarding Associated Bookings.
It is not possible to book separate packages/price tiers but still sit together, in part because each package is generally on a different deck.
You are able to explore the boat beyond the deck you are seated on, including the unobstructed observation deck at the top level with 360 degree views.