The most common outboard hydraulic steering cylinder in service as of this writing is probably the front-mount type branded Teleflex Marine, and now made by SeaStar Solutions. This system is reliable, but with age it can leak, usually around the seal glands on the ends where the piston rod enters and exits the cylinder.
On models such as the 5340 or 5345, replacing the seal glands is easy, thanks to the Teleflex HS5157 seal kit. However, bad seals may be a sign of other problems, such as a burr in the piston rod that is damaging the seal. If there are other issues, replace the cylinder ($501.19, go2marine.com) or have it rebuilt by a shop such as Boat Steering Rebuilders ($250, boatsteeringrebuilders.com).