Yes, you can install the water heater’s vent pipe vertically up into the attic and turn into a horizontal run through the side wall of the house.
Attic consideration / hot summers: “For water heaters in locations with high ambient temperatures (above 100°F) it is recommended that CPVC or ABS pipe and fittings be used." Attic temps may reach way over 100°F during the summer.
General for vent pipe installation: "All vent (exhaust) pipes must be pitched a minimum of a 1/4” per foot back to the water heater to allow drainage of condensation.
Hot summer will not affect the amount of condensation but low temperature will definitely increase its amount so make sure to monitor condensate disposal assembly periodically.
Just follow (or the installer) the instructions provided at the manual (number of elbows and pipe dimensions / lengths are extremely important) and you should be OK. Don’t forget to provide sufficient amount of combustion air if you choose to use single vent pipe installation / combustion air from interior!
There are many important suggestions in the manual so take your time while reading it.