The SE has no *
telephoto - it has a single wide-angle lens for the back camera. (There are no iPhones with an optical zoom, which is classically defined as a single lens with variable focal length.) However, the second-generation SE does support Portrait Mode. The basic specifications of the wide-angle in iPhone X and the iPhone SE (2nd-generation) are the same (12 mp sensor, f/1.8 aperture), but image processing and other advances (A11 Bionic processor in X, A13 Bionic in the SE) deliver a superior experience in the SE.
While your general goal is to have a "disposable" phone on the river (something I appreciate as a longtime whitewater boater and photographer), something to consider is that iPhone 6 and 6s had no water-resistance, which was introduced with iPhone 7. However, that water-resistance does degrade over time, so I wouldn't count on it being all that effective in a several year-old used phone. Basically, the older the phone you carry, the greater the chance that it will succumb to moisture exposure.
I carry my "regular" iPhone on the river, in a water-resistant vinyl pouch. Shooting through that plastic degrades image quality somewhat, but I consider it a reasonable compromise. I can still remove it from the pouch if conditions allow.
Back to the overall choice of "cameras." It's a reasonable assumption that every new model of iPhone has a better camera/camera features than the previous model. Sometimes that improvement is substantial, sometimes less-so. Starting with iPhone 7 Plus the "premium" iPhone models have multiple rear cameras (again, not a zoom, but multiple physical cameras with independent image sensors).
The Apple web site has a technical specifications section where you can look up the specifics of each model.
https://support.apple.com/specs/iphone . There's also a neat little app called Mactracker available in iOS and Mac App Stores that catalogs the specs of nearly every Apple product ever made. My only complaint is that it (like the Apple web site) does not provide all the camera specifics a photographer may desire - especially, no focal lengths for the various lenses, just "wide-angle," "telephoto," etc.
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* The term "telephoto" is worth quibbling over. Classically (prior to smartphones), "telephoto" was a lens of greater focal length than a "normal" lens - normal being defined as a focal length that more or less mimics the perspective of human vision. In 35mm photography "normal" was in the range of 45mm - 55mm, the lens that came as standard equipment on most cameras. A focal length of 35mm or less was considered wide-angle and generally 85mm or more was sold as telephoto. For a variety of good reasons, smartphone makers decided that a wide-angle lens would be standard equipment (the new "normal") and what a 35mm photographer would consider "normal" is now called telephoto. The "longest" telephoto available in an iPhone is in the current iPhone 12 Pro Max - it's equivalent to a 65mm lens on a 35mm camera - barely longer than the old "normal." In comparison to the 26mm-equivalent wide-angle and 13mm-equivalent ultra-wide-angle it's certainly a "zoom-in," but as someone who owns a 100mm-300mm-equivalent zoom for my "good" camera (2x-6x zoom relative to a 50mm "normal" lens) and occasionally rents 600mm-equivalent lenses... 65mm isn't telephoto, although still very helpful compared to shooting only with a wide-angle.