A more comprehensive reference than Waterhouse is the book Representations of Algebraic Groups by J.C. Jantzen (2nd ed., AMS, 2003). Though he aims after a while at prime characteristic, his foundational material in Part I is much more general and often follows the treatment in the older book (in French) by Demazure-Gabriel. Anyway, the appropriate reference is section I.2, especially subsection 2.8.
Jantzen mostly works here over an arbitrary field (sometimes even over a commutative ring), using the language of group schemes and Hopf algebras. You may or may not need the full generality of what he does, but for instance these foundations are needed for the theory of Frobenius kernels in prime characteristic.