For cracks less than 1/8 inch wide, acrylic or epoxy glue will repair a hot tub equally well. First, you should clean the surfaces with isopropyl alcohol to ensure proper adhesion.
For short, straight cracks, try a more fluid glue with a viscosity close to that of water. For longer, irregular cracks, a viscous glue can fill spaces with the help of a smoothing stick.
The Weld-On line of acrylic adhesives offers a range of formulas. On the epoxy resin side, make sure that the formula you use is a "marine-grade" epoxy specifically.
Popular marine-grade epoxies include West System Epoxy, Evercoat Epoxy and PC-11 Marine Epoxy.
When a crack's width exceeds 1/8 inch, the glue color becomes important. The acrylic glue Plast-Aid is formulated specially for the convenient addition of retail acrylic paint.
Beyond this, you may want to invest fiberglass screening to form a base underneath the glue site. If you are dealing with multiple hot tub fragments, the presence of an underlying screen can help prevent shifting during the drying process.
No matter how well you smooth out the glue initially, uneven surfaces are inevitable. To correct this, have a few sheets of extremely fine-grit sandpaper on hand.