Naval intelligence plays a critical role in multi-domain operations by identifying and tracking vessels of interest, especially suspected "dark ships" operating in an emissions-controlled (EMCON) state. While applying machine learning (ML) to maritime satellite imagery could enable an automated open-ocean search capability for dark ships, ensuring the robustness of ML models to environmental variations in the maritime domain remains a challenge because training sets do not encapsulate all possible environmental conditions. To address the challenge of unsupervised domain adaptation (UDA) in ship classification, i.e. transferring a ML model from a labeled source domain to an unlabeled target domain, we propose employing combinations of semi-supervised learning (SSL) techniques with standalone UDA approaches. Specifically, we incorporate combinations of FixMatch, minimum class confusion, gradient reversal, and mixup augmentation into the standard cross-entropy supervised loss function. These interventions were compared in two domain shift settings, one in which the source and target domains are both comprised of simulated data, and another in which the source domain consists of only simulated data, and the target domain consists of only real data. Experimental results comparing the combinations of interventions to a regularized fine-tuning baseline demonstrate that the greatest improvements in model robustness were achieved when combinations of our SSL strategy (FixMatch) and UDA algorithms were incorporated into training.However, ensuring the robustness of ML models for maritime imagery remains a challenge. We define "robustness" as the performance of a ML model on data generated from a counterfactually-altered version of the data-generating process. 5 In maritime imagery, the data-generating process is defined by numerous environmental and operating factors that produce non-i.i.d. data. For instance, the brightness and texture of the ocean's surface under a clear sky is a complex function of the viewing angle, solar angle, and wind speed. 6 Collecting training data over the full range of conditions can be difficult, so datasets may be biased toward ocean conditions with clear skies and calm seas. Furthermore, the observable features of a ship and its wake can change as a function its velocity relative to the waves, sun, and viewpoint, 7 and collected datasets may not be able to sufficiently sample this space.A potential "robustness tactic", 5 or intervention, to improve maritime image classifier robustness is to employ synthetic training data that accommodates physics-based models for the relevant optics and ocean dynamics that define the data-generating process. However, even the most sophisticated models cannot employ all of the relevant factors that impact the imagery (e.g. clouds, biologics). Naively training on simulated data can impose a different bias in the ML model than if it was trained on "cherry-picked" real-world data. Recently, many unsupervised domain adaptation (UDA) techniques 8,9 have been propos...