The Jones Act was passed 97 years ago to protect US shipping within the US from foreign-made ships. I doubt I ever would have supported such an act, but at least back then there were plenty of US-made ships to fulfill the demand. Despite the Jones Act, the US shipping industry has collapsed in the last century so that the number of such ships is far below demand in normal circumstances, so that intra-US shipping costs are far higher than those outside the US. Puerto Rico was covered by he Jones Act and remains so.
After Hurricanes Harvey and Irma the Jones Act was temporarily suspended for Texas, Louisiana, and Florida on orders of President Trump, going through the Department of Homeland Security. The Jones Act is not being suspended for Puerto Rico in the wake of Hurrican Maria, although damage to PR seems to be far greater than what happened on the mainland during Harvey and Irma (with those areas also accessible to supplies and aid by ground transportation, not relying nearly as much on ocean shipping). The supposed reason is that PR's ports are damaged, which is certainly the case, but even if suspending the Jones Act will only slightly speed up deliveries, it will certainly reduce the costs of supplies, allowing cheaper natural gas from Pennsylvania in place of more expensive oil from Venezuela, for example.
Which brings us to the worse then usual hypocrisy on the part of our president. While he has been all worked up over football players kneeling and moved to get aid to Texas and Florida as rapidly as possible while expressing lots of sympathetic sentiments for the victims in those states, his initial reaction to Hurricane Maria, after several days delay, was to talk about how bad their infrastructure was before the hurricane and how they have a massive debt situation. Of course, if he were really concerned about helping them, he could suspend their debt, but at a minimum, given that he is aware that they are poor and debt ridden, on top of having 80% of their crops destroyed and all their power out among other problems, he is insisting that they pay top dollar on supplies brought in by water, where almost all supplies will come. His refusal to suspend the Jones Act for Puerto Rico after having done so for mainland US territories is far worse than the usual hypocrisy from any president, even this far more hypocritical than pretty much all others one.