Indeed K&N 'dry' filters, although any 'dry' type automotive filter will be VASTLY superior to the OEM Yanmar air intake 'filter'. That OEM filter is NOT a filter, but an intake 'silencer' placed there to attenuate *air intake noise* generated by the engine !!!!!!!.
Downside of the K&N filter is that such a company assembles the filter media into 'proprietary shapes' - meaning that the only possible supplier of the filter 'cartridge' will be K&N. An alternative is to go to an 'automotive junkyard", find one of the smallest late model wrecks, and simply get the ('flat pack' pleated) filter assembly /housing and all the associated connection hosing from the vehicle, then go to an automotive supply (or better, go to your local 'tinsmith') and find/make the correct adapter to fit your engines air intake; standard auto supply stores (the one that 'mechanics' use) sell large diameter, lightweight 'flexible metal hose' at economical prices. Such 'flat pack' filters can be mounted *anywhere*, will be greatly oversized for your application, the replacement elements will be common and will be commonly available for a long time (17 to 20 years for 'auto stuff') and many auto filter 'assemblers'/manufacturers products will 'fit'.
Dont worry about OEM requirements, the OEM diesel makers dont make filters, they BUY their filters from filter manufacturers as rebranded products and significantly overcharge the end user. The filter media used in automotive air, oil, etc. filters is made by essentially ONE sole source in the entire world, the filter manufactures all buy from virtually the same single source. Thinking that OEM filters are somehow better is 'hoooey', the only real difference between OEM filters and 'aftermarket' filters is PRICE and markup for the OEM distributor.
Oversizing (the effective surface area) such filters is a VERY good idea, as the SLOWER the particle laden air passes through the dry filter media the greater the effective filtration and particle removal.
Are (true) air filters needed on marine diesels that run in a 'clean' environment such as in the ocean or large lakes???? You betcha, as normal 'clean' ambient air can typically have up to 10000 particles per cu. ft. @ 0.1µM ... and then add the debris from distant volcanoes, dust storms, etc. etc. that can circle the earth on the jet stream, etc. .... and you can have quite a storm of particles being aspirated by your engine coming from 'clear blue sky' - particles that are invisible to a foam 'silencer'.
With Filtration BIG (surface area) size is ALWAYS better.