Interactions with marine vegetation can disrupt unmanned underwater vehicle missions. Very little information is publicly available about the mechanisms causing these interactions or the consequences of them. This article compares the interactions between three different style underwater vehicles and two different types of marine vegetation. Similar test setups and procedures were used to allow for the direct comparison between REMUS-100, BlueROV2, and GhostSwimmer vehicles. Experimental test runs were conducted at different vegetation densities using either synthetic eelgrass or synthetic giant kelp. The resulting interactions depended on the vegetation type, vegetation density, propulsion mechanism of the vehicle, and vehicle geometry. Synthetic giant kelp caused a multitude of interactions with the dominant ones being interference, blockage, and entanglement with the vehicle body. Collectively, these three interactions occurred 84% of the time on giant kelp runs where an interaction was observed. Eelgrass caused propeller entanglement and even body blockage depending on the unmanned underwater vehicle geometry. In high density eelgrass, we observed that a spinning propulsion mechanism experiences entanglement 100% of the time when at low speed. The use of an oscillating tail for propulsion coupled with a completely streamlined body appears to successfully mitigate adverse marine vegetation interactions.