Again, this whole part of the chapter makes no sense to me. I honestly feel like Cassandra Clare is doing her damnest to confuse the fuck out of all of us. First of all, they couldn’t just rent or steal a boat?
And yes, Jace talked about Valentine having wards on the boat. However, Jace was able to actually get on the boat with no problem (except with the encounter the Greater Demon), but suddenly Valentine has wards. Or maybe Valentine just keyed the wards to let Jace enter, but that doesn’t make sense because it looked like Valentine was really surprised to find Jace on his boat.
Either way, nothing makes any logical sense and Clare continues to be a horrible hack.