Yes, travel insurance is worth carrying for a trip to Semarang, and the honest reason has as much to do with water as anything else. This Central Java port city sits low against the Java Sea and is genuinely prone to tidal flooding and land subsidence, on top of the usual reasons any visitor to Indonesia should carry cover. Most travelers pass through Semarang for business or as a stop between other Central Java destinations rather than as a standalone beach or resort trip.
Semarang's low-lying streets, including parts of Kota Lama, the city's Dutch colonial old town, flood periodically even outside a major storm, and the wet monsoon season, roughly November through March, makes this worse with heavy rain layered on top of the tidal issue. Trip-delay cover is worth checking if flooding disrupts transport during your visit, and it is worth watching local advisories once you arrive rather than assuming the dry-season pattern still applies.
Mount Ungaran rises just behind the city and is a genuine feature of the local landscape for anyone hiking or touring nearby, though it is not currently a source of acute volcanic activity; general trekking cover is still worth confirming if hill walking around it is part of your plans, since footing and weather on its lower slopes can turn quickly during the wet season.
Semarang has local hospitals for routine care, but anything serious is typically stabilized here and then referred to Jakarta, Indonesia's capital and a genuinely larger medical center, or in more complex cases onward to Singapore, the region's real specialist hub. Confirm your policy covers treatment and transfer between Indonesian cities and international evacuation to Singapore if needed, and keep any clinic paperwork, flood-related transport receipts, and treatment invoices on hand for a claim, since a referral spanning two countries generally needs more documentation than a routine local visit.