runaway values with robot_localization, no velocity sensors

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I have a boat with only two sensors: a VectorNav IMU and a Marvelmind indoor GPS unit. Notably, there are no sensors that can measure velocity, and ekf_localization_node quickly thinks that the boat is going 100+ m/s for some reason—even with no position changes. I'm running Ubuntu 20.04 with ROS Noetic. My launch file is:

<launch>

<param name="use_sim_time" value="true" />
<node name="base_link_to_hedgehog" pkg="tf" type="static_transform_publisher" args="0 0 0 0 0 0 base_link hedgehog 50" />


<node name="odom_to_base_link" pkg="tf" type="static_transform_publisher" args="0 0 0 0 0 0 odom base_link 10" />

<!-- https://roscon.ros.org/2015/presentations/robot_localization.pdf -->
<node name="indoor_ukf" pkg="robot_localization" type="ekf_localization_node">
    <param name="world_frame" value="map" />
    <param name="map_frame" value="map" />
    <param name="odom_frame" value="odom" />

    <param name="frequency" value="40" />

    <param name="pose0" value="/indoor_gps"/>
    <param name="pose0_relative" value="false" />
    <param name="pose0_differential" value="false" />
    <param name="pose0_queue_size" value="100" /> 
    <rosparam param="pose0_config">[true,  true,  true,
                                    false, false, false,
                                    false, false, false,
                                    false, false, false,
                                    false, false, false]</rosparam>

    <param name="imu0" value="/vectornav/IMU"/>
    <param name="imu0_differential" value="false" />
    <param name="imu0_relative" value="false" />
    <param name="imu0_queue_size" value="1000" />
    <rosparam param="imu0_config"> [false, false, false,
                                    true,  true,  true,
                                    false, false, false,
                                    false, false, false,
                                    false, false, false]</rosparam>

    <param name="imu0_remove_gravitational_acceleration" value="true" />

    <param name="two_d_mode" value="false" />

    <rosparam param="initial_state">[0.0, 0.0, 0.0,
                                         0.0, 0.0, 0.0,
                                         0.0, 0.0, 0.0,
                                         0.0, 0.0, 0.0,
                                         0.0, 0.0, 0.0]</rosparam>

    <param name="print_diagnostics" value="true"/>
    <param name="publish_tf" value="true"/>
</node>
</launch>

I had a setup with two localization nodes replacing the static transform from odom to base_link but removed it for simplicity since the issue persists in both configurations—and it seems not to be necessary anyways ("If all you have is IMU and GPS data, then you don't need to run two EKFs, as it won't really help you").

A rosbag is available here.