In Jainism, traditionally, crossing the ocean and traveling overseas (called "samudra langhana") has been discouraged. This prohibition is rooted in several religious and ethical considerations:

1. Ahimsa (Non-violence): Jainism places utmost emphasis on ahimsa, or non-violence, towards all living beings. Oceans and large bodies of water are believed to contain countless microscopic and macroscopic living beings (aquatic organisms). Traveling by sea could inadvertently harm or kill these beings, which goes against Jain principles.

1. Difficulty in Observing Religious Practices: Overseas travel, especially in ancient times, made it difficult to maintain the strict Jain dietary rules and daily rituals (such as pratikraman, samayik, etc.). Access to pure vegetarian food, clean water, and places suitable for meditation and religious observances was uncertain.

1. Monastic Rules: For Jain monks and nuns, the rules are even stricter. They are not allowed to travel over water because it is impossible to walk mindfully and avoid harming living beings in the water. They are also required to avoid places where they cannot adhere to their vows.

1. Cultural and Community Factors: Historically, the Jain community developed social norms to discourage overseas travel to preserve religious and cultural identity. There was a belief that those who went overseas would adopt foreign customs, making it difficult to uphold Jain values.

1. Scriptural References: Classical Jain texts, such as the Shravakachara and other conduct manuals, mention the avoidance of sea voyages for laypersons (shravaks) as well, although the prohibition is more explicit for ascetics.

Modern Perspective: While the traditional restriction remains respected by many, especially among orthodox Jains and monks, some contemporary lay Jains do travel overseas for business, education, or other reasons, taking care to follow Jain principles as much as possible.

In summary, the restriction comes from the core Jain values of non-violence, purity, and the practical challenges of overseas travel in maintaining religious observances.