While there might be some institutions with the capability to make these transgenic watermelon and coconut plants for you, that does not mean that you would be able to actually plant them out. First, the institution would need to have a Biological Use Authorization to work with recombinant DNA to make the vectors to transfer the genes. Then they would need to be able to do the tissue culture required to transfer the genes and regenerate whole plants again, which can sometimes be difficult. Then a number of these plants (between 20 and 100) might need to be grown to identify ones that have successfully incorporated the genes and express the desired trait. This would have to done in a greenhouse with proper containment to prevent the plants (or any of their parts, e.g., pollen) from the outdoor environment. Generally, you would grow them there for a generation or two to be sure that the trait is stable, but this would clearly be a problem for coconut. If you then wanted to grow them outside, you would need additional permits from the USDA, an isolated field, and the ability to monitor the field for two more years afterwards to be sure that there are no volunteer plants (at least this would be easy for coconut). Then, if you wanted to sell or distribute these plants, you would need to submit an application to the USDA-APHIS for review to allow them to decide whether there is any risk of the plants becoming plant pests due to the gene you added and to the FDA to be sure that they would be safe to eat. This review can take anywhere from a few years up to a decade or more, based on recent cases, and could cost a minimum of $1-10 million up to much more to generate the required data, depending on what the genes are and whether you also want to send the plants to or sell them in other countries.
In short, while the technology to do what you ask is actually pretty easy to accomplish these days inside a research setting, the regulatory requirements that must be met in order to create, characterize, deregulate and finally release such plants to the environment make it virtually impossible to do without very deep pockets and a lot of time.